diff --git a/internal/cgen/base/io-private.h b/internal/cgen/base/io-private.h
index d13e055..e19940c 100644
--- a/internal/cgen/base/io-private.h
+++ b/internal/cgen/base/io-private.h
@@ -34,7 +34,7 @@
 
 static inline uint32_t  //
 wuffs_base__io_writer__copy_n_from_history(uint8_t** ptr_iop_w,
-                                           uint8_t* io0_w,
+                                           uint8_t* io1_w,
                                            uint8_t* io2_w,
                                            uint32_t length,
                                            uint32_t distance) {
@@ -42,7 +42,7 @@
     return 0;
   }
   uint8_t* p = *ptr_iop_w;
-  if ((size_t)(p - io0_w) < (size_t)(distance)) {
+  if ((size_t)(p - io1_w) < (size_t)(distance)) {
     return 0;
   }
   uint8_t* q = p - distance;
@@ -78,11 +78,11 @@
 // wuffs_base__io_writer__copy_n_from_history function above, but has stronger
 // pre-conditions. The caller needs to prove that:
 //  - distance >  0
-//  - distance <= (*ptr_iop_w - io0_w)
+//  - distance <= (*ptr_iop_w - io1_w)
 //  - length   <= (io2_w      - *ptr_iop_w)
 static inline uint32_t  //
 wuffs_base__io_writer__copy_n_from_history_fast(uint8_t** ptr_iop_w,
-                                                uint8_t* io0_w,
+                                                uint8_t* io1_w,
                                                 uint8_t* io2_w,
                                                 uint32_t length,
                                                 uint32_t distance) {
diff --git a/internal/cgen/builtin.go b/internal/cgen/builtin.go
index a2e96be..e2fcff0 100644
--- a/internal/cgen/builtin.go
+++ b/internal/cgen/builtin.go
@@ -137,7 +137,7 @@
 		return nil
 
 	case t.IDCanUndoByte:
-		b.writes("(iop_a_src > io0_a_src)")
+		b.writes("(iop_a_src > io1_a_src)")
 		return nil
 
 	case t.IDCountSince:
diff --git a/internal/cgen/cgen.go b/internal/cgen/cgen.go
index 2b1bac1..670c0e9 100644
--- a/internal/cgen/cgen.go
+++ b/internal/cgen/cgen.go
@@ -68,22 +68,22 @@
 // reading or writing the next byte (and advancing the stream) is essentially
 // "etc = *iop_a_src++" or "*io_a_dst++ = etc".
 //
-// The other two prefixes, giving names like io0_etc and io2_etc, are
+// The other two prefixes, giving names like io1_etc and io2_etc, are
 // auxilliary pointers: lower and upper inclusive bounds. As an iop_etc pointer
 // advances, it cannot advance past io2_etc. In the rarer case that an iop_etc
-// pointer retreats, undoing a read or write, it cannot retreat past io0_etc.
+// pointer retreats, undoing a read or write, it cannot retreat past io1_etc.
 //
 // At the start of a function, these pointers are initialized from an
 // io_buffer's fields (ptr, ri, wi, len). For an io_reader:
-//  - io0_etc = ptr + ri
+//  - io1_etc = ptr + ri
 //  - iop_etc = ptr + ri
 //  - io2_etc = ptr + wi
 // and for an io_writer:
-//  - io0_etc = ptr + wi
+//  - io1_etc = ptr + wi
 //  - iop_etc = ptr + wi
 //  - io2_etc = ptr + len
 const (
-	io0Prefix = "io0_" // Lower bound.
+	io1Prefix = "io1_" // Lower bound.
 	io2Prefix = "io2_" // Upper bound.
 	iopPrefix = "iop_" // Pointer.
 )
diff --git a/internal/cgen/data.go b/internal/cgen/data.go
index de95d33..13e1aea 100644
--- a/internal/cgen/data.go
+++ b/internal/cgen/data.go
@@ -163,9 +163,9 @@
 	""
 
 const baseIOPrivateH = "" +
-	"// ---------------- I/O\n\nstatic inline uint64_t  //\nwuffs_base__io__count_since(uint64_t mark, uint64_t index) {\n  if (index >= mark) {\n    return index - mark;\n  }\n  return 0;\n}\n\nstatic inline wuffs_base__slice_u8  //\nwuffs_base__io__since(uint64_t mark, uint64_t index, uint8_t* ptr) {\n  if (index >= mark) {\n    return wuffs_base__make_slice_u8(ptr + mark, index - mark);\n  }\n  return wuffs_base__make_slice_u8(NULL, 0);\n}\n\nstatic inline uint32_t  //\nwuffs_base__io_writer__copy_n_from_history(uint8_t** ptr_iop_w,\n                                           uint8_t* io0_w,\n                                           uint8_t* io2_w,\n                                           uint32_t length,\n                                           uint32_t distance) {\n  if (!distance) {\n    return 0;\n  }\n  uint8_t* p = *ptr_iop_w;\n  if ((size_t)(p - io0_w) < (size_t)(distance)) {\n    return 0;\n  }\n  uint8_t* q = p - distance;\n  size_t n = (size_t)(io2_w - p);\n  if ((size_t)(length) > n) {\n    length = (uint32_t)(n);\n  } else {\n" +
+	"// ---------------- I/O\n\nstatic inline uint64_t  //\nwuffs_base__io__count_since(uint64_t mark, uint64_t index) {\n  if (index >= mark) {\n    return index - mark;\n  }\n  return 0;\n}\n\nstatic inline wuffs_base__slice_u8  //\nwuffs_base__io__since(uint64_t mark, uint64_t index, uint8_t* ptr) {\n  if (index >= mark) {\n    return wuffs_base__make_slice_u8(ptr + mark, index - mark);\n  }\n  return wuffs_base__make_slice_u8(NULL, 0);\n}\n\nstatic inline uint32_t  //\nwuffs_base__io_writer__copy_n_from_history(uint8_t** ptr_iop_w,\n                                           uint8_t* io1_w,\n                                           uint8_t* io2_w,\n                                           uint32_t length,\n                                           uint32_t distance) {\n  if (!distance) {\n    return 0;\n  }\n  uint8_t* p = *ptr_iop_w;\n  if ((size_t)(p - io1_w) < (size_t)(distance)) {\n    return 0;\n  }\n  uint8_t* q = p - distance;\n  size_t n = (size_t)(io2_w - p);\n  if ((size_t)(length) > n) {\n    length = (uint32_t)(n);\n  } else {\n" +
 	"    n = (size_t)(length);\n  }\n  // TODO: unrolling by 3 seems best for the std/deflate benchmarks, but that\n  // is mostly because 3 is the minimum length for the deflate format. This\n  // function implementation shouldn't overfit to that one format. Perhaps the\n  // copy_n_from_history Wuffs method should also take an unroll hint argument,\n  // and the cgen can look if that argument is the constant expression '3'.\n  //\n  // See also wuffs_base__io_writer__copy_n_from_history_fast below.\n  //\n  // Alternatively, or additionally, have a sloppy_copy_n_from_history method\n  // that copies 8 bytes at a time, possibly writing more than length bytes?\n  for (; n >= 3; n -= 3) {\n    *p++ = *q++;\n    *p++ = *q++;\n    *p++ = *q++;\n  }\n  for (; n; n--) {\n    *p++ = *q++;\n  }\n  *ptr_iop_w = p;\n  return length;\n}\n\n// wuffs_base__io_writer__copy_n_from_history_fast is like the\n// wuffs_base__io_writer__copy_n_from_history function above, but has stronger\n// pre-conditions. The caller needs to prove that:\n//  - distance >  " +
-	"0\n//  - distance <= (*ptr_iop_w - io0_w)\n//  - length   <= (io2_w      - *ptr_iop_w)\nstatic inline uint32_t  //\nwuffs_base__io_writer__copy_n_from_history_fast(uint8_t** ptr_iop_w,\n                                                uint8_t* io0_w,\n                                                uint8_t* io2_w,\n                                                uint32_t length,\n                                                uint32_t distance) {\n  uint8_t* p = *ptr_iop_w;\n  uint8_t* q = p - distance;\n  uint32_t n = length;\n  for (; n >= 3; n -= 3) {\n    *p++ = *q++;\n    *p++ = *q++;\n    *p++ = *q++;\n  }\n  for (; n; n--) {\n    *p++ = *q++;\n  }\n  *ptr_iop_w = p;\n  return length;\n}\n\nstatic inline uint32_t  //\nwuffs_base__io_writer__copy_n_from_reader(uint8_t** ptr_iop_w,\n                                          uint8_t* io2_w,\n                                          uint32_t length,\n                                          uint8_t** ptr_iop_r,\n                                          uint8_t* io2_r) {\n  uint8_t* i" +
+	"0\n//  - distance <= (*ptr_iop_w - io1_w)\n//  - length   <= (io2_w      - *ptr_iop_w)\nstatic inline uint32_t  //\nwuffs_base__io_writer__copy_n_from_history_fast(uint8_t** ptr_iop_w,\n                                                uint8_t* io1_w,\n                                                uint8_t* io2_w,\n                                                uint32_t length,\n                                                uint32_t distance) {\n  uint8_t* p = *ptr_iop_w;\n  uint8_t* q = p - distance;\n  uint32_t n = length;\n  for (; n >= 3; n -= 3) {\n    *p++ = *q++;\n    *p++ = *q++;\n    *p++ = *q++;\n  }\n  for (; n; n--) {\n    *p++ = *q++;\n  }\n  *ptr_iop_w = p;\n  return length;\n}\n\nstatic inline uint32_t  //\nwuffs_base__io_writer__copy_n_from_reader(uint8_t** ptr_iop_w,\n                                          uint8_t* io2_w,\n                                          uint32_t length,\n                                          uint8_t** ptr_iop_r,\n                                          uint8_t* io2_r) {\n  uint8_t* i" +
 	"op_w = *ptr_iop_w;\n  size_t n = length;\n  if (n > ((size_t)(io2_w - iop_w))) {\n    n = (size_t)(io2_w - iop_w);\n  }\n  uint8_t* iop_r = *ptr_iop_r;\n  if (n > ((size_t)(io2_r - iop_r))) {\n    n = (size_t)(io2_r - iop_r);\n  }\n  if (n > 0) {\n    memmove(iop_w, iop_r, n);\n    *ptr_iop_w += n;\n    *ptr_iop_r += n;\n  }\n  return (uint32_t)(n);\n}\n\nstatic inline uint64_t  //\nwuffs_base__io_writer__copy_from_slice(uint8_t** ptr_iop_w,\n                                       uint8_t* io2_w,\n                                       wuffs_base__slice_u8 src) {\n  uint8_t* iop_w = *ptr_iop_w;\n  size_t n = src.len;\n  if (n > ((size_t)(io2_w - iop_w))) {\n    n = (size_t)(io2_w - iop_w);\n  }\n  if (n > 0) {\n    memmove(iop_w, src.ptr, n);\n    *ptr_iop_w += n;\n  }\n  return (uint64_t)(n);\n}\n\nstatic inline uint32_t  //\nwuffs_base__io_writer__copy_n_from_slice(uint8_t** ptr_iop_w,\n                                         uint8_t* io2_w,\n                                         uint32_t length,\n                                         w" +
 	"uffs_base__slice_u8 src) {\n  uint8_t* iop_w = *ptr_iop_w;\n  size_t n = src.len;\n  if (n > length) {\n    n = length;\n  }\n  if (n > ((size_t)(io2_w - iop_w))) {\n    n = (size_t)(io2_w - iop_w);\n  }\n  if (n > 0) {\n    memmove(iop_w, src.ptr, n);\n    *ptr_iop_w += n;\n  }\n  return (uint32_t)(n);\n}\n\nstatic inline wuffs_base__empty_struct  //\nwuffs_base__io_reader__set(wuffs_base__io_buffer** o,\n                           wuffs_base__io_buffer* b,\n                           uint8_t** ptr_iop_r,\n                           uint8_t** ptr_io2_r,\n                           wuffs_base__slice_u8 data) {\n  b->data = data;\n  b->meta.wi = data.len;\n  b->meta.ri = 0;\n  b->meta.pos = 0;\n  b->meta.closed = false;\n\n  *o = b;\n  *ptr_iop_r = data.ptr;\n  *ptr_io2_r = data.ptr + data.len;\n\n  wuffs_base__empty_struct ret;\n  ret.private_impl = 0;\n  return ret;\n}\n\nstatic inline wuffs_base__slice_u8  //\nwuffs_base__io_reader__take(uint8_t** ptr_iop_r, uint8_t* io2_r, uint64_t n) {\n  if (n <= ((size_t)(io2_r - *ptr_iop_r))) {\n    uint8_t*" +
 	" p = *ptr_iop_r;\n    *ptr_iop_r += n;\n    return wuffs_base__make_slice_u8(p, n);\n  }\n  return wuffs_base__make_slice_u8(NULL, 0);\n}\n\nstatic inline wuffs_base__empty_struct  //\nwuffs_base__io_writer__set(wuffs_base__io_buffer** o,\n                           wuffs_base__io_buffer* b,\n                           uint8_t** ptr_iop_w,\n                           uint8_t** ptr_io2_w,\n                           wuffs_base__slice_u8 data) {\n  b->data = data;\n  b->meta.wi = 0;\n  b->meta.ri = 0;\n  b->meta.pos = 0;\n  b->meta.closed = false;\n\n  *o = b;\n  *ptr_iop_w = data.ptr;\n  *ptr_io2_w = data.ptr + data.len;\n\n  wuffs_base__empty_struct ret;\n  ret.private_impl = 0;\n  return ret;\n}\n\n" +
diff --git a/internal/cgen/var.go b/internal/cgen/var.go
index 5c6ffe1..fb08a55 100644
--- a/internal/cgen/var.go
+++ b/internal/cgen/var.go
@@ -102,7 +102,7 @@
 
 	if header {
 		b.printf("uint8_t* %s%s = NULL;", iopPrefix, preName)
-		b.printf("uint8_t* %s%s WUFFS_BASE__POTENTIALLY_UNUSED = NULL;", io0Prefix, preName)
+		b.printf("uint8_t* %s%s WUFFS_BASE__POTENTIALLY_UNUSED = NULL;", io1Prefix, preName)
 		b.printf("uint8_t* %s%s WUFFS_BASE__POTENTIALLY_UNUSED = NULL;", io2Prefix, preName)
 	}
 
@@ -112,7 +112,7 @@
 		iopPrefix, preName, preName, preName, i0)
 
 	if header {
-		b.printf("%s%s = %s%s;", io0Prefix, preName, iopPrefix, preName)
+		b.printf("%s%s = %s%s;", io1Prefix, preName, iopPrefix, preName)
 		b.printf("%s%s = %s->data.ptr + %s->%s;",
 			io2Prefix, preName, preName, preName, i1)
 
@@ -296,7 +296,7 @@
 			b.printf("wuffs_base__io_buffer %s%s WUFFS_BASE__POTENTIALLY_UNUSED = "+
 				"wuffs_base__null_io_buffer();\n", uPrefix, name)
 			preName := vPrefix + name
-			// TODO: io0_etc variables?
+			// TODO: io1_etc variables?
 			b.printf("uint8_t* %s%s WUFFS_BASE__POTENTIALLY_UNUSED = NULL;\n", iopPrefix, preName)
 			b.printf("uint8_t* %s%s WUFFS_BASE__POTENTIALLY_UNUSED = NULL;\n", io2Prefix, preName)
 		}
diff --git a/release/c/wuffs-unsupported-snapshot.c b/release/c/wuffs-unsupported-snapshot.c
index cd1275a..0a6d4d0 100644
--- a/release/c/wuffs-unsupported-snapshot.c
+++ b/release/c/wuffs-unsupported-snapshot.c
@@ -4484,7 +4484,7 @@
 
 static inline uint32_t  //
 wuffs_base__io_writer__copy_n_from_history(uint8_t** ptr_iop_w,
-                                           uint8_t* io0_w,
+                                           uint8_t* io1_w,
                                            uint8_t* io2_w,
                                            uint32_t length,
                                            uint32_t distance) {
@@ -4492,7 +4492,7 @@
     return 0;
   }
   uint8_t* p = *ptr_iop_w;
-  if ((size_t)(p - io0_w) < (size_t)(distance)) {
+  if ((size_t)(p - io1_w) < (size_t)(distance)) {
     return 0;
   }
   uint8_t* q = p - distance;
@@ -4528,11 +4528,11 @@
 // wuffs_base__io_writer__copy_n_from_history function above, but has stronger
 // pre-conditions. The caller needs to prove that:
 //  - distance >  0
-//  - distance <= (*ptr_iop_w - io0_w)
+//  - distance <= (*ptr_iop_w - io1_w)
 //  - length   <= (io2_w      - *ptr_iop_w)
 static inline uint32_t  //
 wuffs_base__io_writer__copy_n_from_history_fast(uint8_t** ptr_iop_w,
-                                                uint8_t* io0_w,
+                                                uint8_t* io1_w,
                                                 uint8_t* io2_w,
                                                 uint32_t length,
                                                 uint32_t distance) {
@@ -6396,11 +6396,11 @@
   uint32_t v_already_full = 0;
 
   uint8_t* iop_a_dst = NULL;
-  uint8_t* io0_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_dst) {
     iop_a_dst = a_dst->data.ptr + a_dst->meta.wi;
-    io0_a_dst = iop_a_dst;
+    io1_a_dst = iop_a_dst;
     io2_a_dst = a_dst->data.ptr + a_dst->data.len;
     if (a_dst->meta.closed) {
       io2_a_dst = iop_a_dst;
@@ -6513,11 +6513,11 @@
   wuffs_base__status v_status = NULL;
 
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -6657,22 +6657,22 @@
   uint32_t v_n_copied = 0;
 
   uint8_t* iop_a_dst = NULL;
-  uint8_t* io0_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_dst) {
     iop_a_dst = a_dst->data.ptr + a_dst->meta.wi;
-    io0_a_dst = iop_a_dst;
+    io1_a_dst = iop_a_dst;
     io2_a_dst = a_dst->data.ptr + a_dst->data.len;
     if (a_dst->meta.closed) {
       io2_a_dst = iop_a_dst;
     }
   }
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -6831,11 +6831,11 @@
   uint32_t v_b3 = 0;
 
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -7334,22 +7334,22 @@
   uint32_t v_hdist = 0;
 
   uint8_t* iop_a_dst = NULL;
-  uint8_t* io0_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_dst) {
     iop_a_dst = a_dst->data.ptr + a_dst->meta.wi;
-    io0_a_dst = iop_a_dst;
+    io1_a_dst = iop_a_dst;
     io2_a_dst = a_dst->data.ptr + a_dst->data.len;
     if (a_dst->meta.closed) {
       io2_a_dst = iop_a_dst;
     }
   }
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -7568,7 +7568,7 @@
 label_0_break:;
   while (v_n_bits >= 8) {
     v_n_bits -= 8;
-    if (iop_a_src > io0_a_src) {
+    if (iop_a_src > io1_a_src) {
       (iop_a_src--, wuffs_base__make_empty_struct());
     } else {
       status = wuffs_deflate__error__internal_error_inconsistent_i_o;
@@ -7623,22 +7623,22 @@
   uint32_t v_hdist = 0;
 
   uint8_t* iop_a_dst = NULL;
-  uint8_t* io0_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_dst) {
     iop_a_dst = a_dst->data.ptr + a_dst->meta.wi;
-    io0_a_dst = iop_a_dst;
+    io1_a_dst = iop_a_dst;
     io2_a_dst = a_dst->data.ptr + a_dst->data.len;
     if (a_dst->meta.closed) {
       io2_a_dst = iop_a_dst;
     }
   }
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -8224,11 +8224,11 @@
   uint16_t v_lm1_a = 0;
 
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -8369,7 +8369,7 @@
   if (self->private_impl.f_read_from_return_value != 2) {
     while (v_n_bits >= 8) {
       v_n_bits -= 8;
-      if (iop_a_src > io0_a_src) {
+      if (iop_a_src > io1_a_src) {
         (iop_a_src--, wuffs_base__make_empty_struct());
       } else {
         self->private_impl.f_read_from_return_value = 4;
@@ -8402,11 +8402,11 @@
   uint64_t v_n = 0;
 
   uint8_t* iop_a_dst = NULL;
-  uint8_t* io0_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_dst) {
     iop_a_dst = a_dst->data.ptr + a_dst->meta.wi;
-    io0_a_dst = iop_a_dst;
+    io1_a_dst = iop_a_dst;
     io2_a_dst = a_dst->data.ptr + a_dst->data.len;
     if (a_dst->meta.closed) {
       io2_a_dst = iop_a_dst;
@@ -8830,11 +8830,11 @@
   wuffs_base__status status = NULL;
 
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -9086,11 +9086,11 @@
   uint8_t v_flags = 0;
 
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -9229,11 +9229,11 @@
   uint8_t v_lw = 0;
 
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -9425,11 +9425,11 @@
   uint8_t v_block_type = 0;
 
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -9543,11 +9543,11 @@
   uint32_t v_i = 0;
 
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -9614,11 +9614,11 @@
   uint32_t v_argb = 0;
 
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -9823,11 +9823,11 @@
   uint8_t v_label = 0;
 
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -9917,11 +9917,11 @@
   uint8_t v_block_size = 0;
 
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -9993,11 +9993,11 @@
   bool v_is_xmp = false;
 
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -10242,11 +10242,11 @@
   uint16_t v_gc_duration_centiseconds = 0;
 
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -10369,11 +10369,11 @@
   wuffs_base__status status = NULL;
 
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -10549,11 +10549,11 @@
   uint8_t v_lw = 0;
 
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -10750,11 +10750,11 @@
   wuffs_base__slice_u8 v_uncompressed = {0};
 
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -11230,22 +11230,22 @@
   uint32_t v_decoded_length_want = 0;
 
   uint8_t* iop_a_dst = NULL;
-  uint8_t* io0_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_dst) {
     iop_a_dst = a_dst->data.ptr + a_dst->meta.wi;
-    io0_a_dst = iop_a_dst;
+    io1_a_dst = iop_a_dst;
     io2_a_dst = a_dst->data.ptr + a_dst->data.len;
     if (a_dst->meta.closed) {
       io2_a_dst = iop_a_dst;
     }
   }
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }
 
@@ -11702,22 +11702,22 @@
   uint64_t v_mark = 0;
 
   uint8_t* iop_a_dst = NULL;
-  uint8_t* io0_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_dst WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_dst) {
     iop_a_dst = a_dst->data.ptr + a_dst->meta.wi;
-    io0_a_dst = iop_a_dst;
+    io1_a_dst = iop_a_dst;
     io2_a_dst = a_dst->data.ptr + a_dst->data.len;
     if (a_dst->meta.closed) {
       io2_a_dst = iop_a_dst;
     }
   }
   uint8_t* iop_a_src = NULL;
-  uint8_t* io0_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
+  uint8_t* io1_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   uint8_t* io2_a_src WUFFS_BASE__POTENTIALLY_UNUSED = NULL;
   if (a_src) {
     iop_a_src = a_src->data.ptr + a_src->meta.ri;
-    io0_a_src = iop_a_src;
+    io1_a_src = iop_a_src;
     io2_a_src = a_src->data.ptr + a_src->meta.wi;
   }