GenEx: add $<LIST:FILTER> PREDICATE filter and REGEX keyword Let $<LIST:FILTER> select elements by an arbitrary condition: PREDICATE keeps or drops each element according to a <body> evaluated with $<_0> bound, complementing the existing regular-expression form. Also accept an explicit REGEX keyword before the pattern so the regex and predicate forms read symmetrically; the bare pattern keeps working. Issue: #27892