Work towards implementing :squash for &Str.trans.
Let tr/// support adverbs, implement some.

Adverbs for tr/// work and are implemented as named
parameters for Str.trans, which obviously supports the
same ones. :squash and :delete are implemented decently (IMHO),
while :complement isn't at all at present.
More adverbs support for .trans.
Implement closures as replacement for .trans.