Allow first=None for add_implicit_resolver.
diff --git a/lib/yaml/resolver.py b/lib/yaml/resolver.py
index 2329c63..7e580e9 100644
--- a/lib/yaml/resolver.py
+++ b/lib/yaml/resolver.py
@@ -25,6 +25,8 @@
def add_implicit_resolver(cls, tag, regexp, first):
if not 'yaml_implicit_resolvers' in cls.__dict__:
cls.yaml_implicit_resolvers = cls.yaml_implicit_resolvers.copy()
+ if first is None:
+ first = [None]
for ch in first:
cls.yaml_implicit_resolvers.setdefault(ch, []).append((tag, regexp))
add_implicit_resolver = classmethod(add_implicit_resolver)
