Returns a deep copy of a REGEX (see COPY-REGEX) and
optionally removes embedded REGISTER objects if possible and if the
special variable REMOVE-REGISTERS-P is true.
| remove-registers < alternation > |
| remove-registers < branch > |
| remove-registers < lookahead > |
| remove-registers < lookbehind > |
| remove-registers < regex > |
| remove-registers < register > |
| remove-registers < repetition > |
| remove-registers < seq > |
| remove-registers < standalone > |