A VIP is a common syntax error, for which a good error message should be given.
Everything that is allowed in a structure but forbidden in a signature, or vice-versa, is a VIP. For instance, writing:
exception A = B
is allowed in a structure, but forbidden in a signature. (Here, we might
wish to make the error message depend on the lookahead token; the token
= suggests that the user confuses a structure and a signature.)
Similarly, writing struct where sig is expected, or vice-versa, is
probably a common mistake.