For completeness, here's my new spec:
* The priming operator is defined thus: e' == e with all its variables primed, except those enclosed in a CAPTURE block.
* It is level-incorrect to write CAPTURE e, if e does not have level at most 1
* When appearing in an unprimed _expression_, CAPTURE e has the same meaning as e.
The spec does not define the syntactic scope (precedence) of CAPTURE, but that is orthogonal. Perhaps it would be better to use some symbol or a special bracketing to delimit the captured _expression_, rather than use the word CAPTURE, which seems like an operator.