RestrictedPerm( perm, list )
RestrictedPerm returns the new permutation new that operates on the
points in the list list in the same way as the permutation perm, and
that fixes those points that are not in list. list must be a list of
positive integers such that for each i in list the image
i^perm is also in list, i.e., it must be the union of cycles of
perm.
gap> RestrictedPerm( (1,2,3)(4,5), [4,5] );
(4,5)
GAP 3.4.4