Semantic action's type.
rename renaming_env phi a builds the action
let x1 = x1' and ... xn = xn' in a if
(x1, x1') ... (xn, xn').
renaming_env is used to correctly replace $startpos/$endpos
present in the semantic action.
A semantic action might be the inlining of several others. The
filenames of the different parts are given by
filenames a. This
can be used, for instance, to check whether all parts come from
the standard library.