Conversion to negate an integer literal of type :int.
The call INT_NEG_CONV `--c`, where c is an integer literal of type
:int, returns the theorem |- --c = d where d is the canonical integer
literal that is equal to c's negation. The literal c may be of the form
&n or -- &n (with nonzero n in the latter case) and the result will be of
the same form.
Fails if applied to a term that is not the negation of one of the permitted
forms of integer literal of type :int.
The related function REAL_RAT_NEG_CONV subsumes this functionality, also
applying to rational literals. Unless the restriction to integers is desired or
a tiny efficiency difference matters, it should be used in preference.