Conversion to perform multiplication on two integer literals of type :int.
The call INT_MUL_CONV `c1 * c2` where c1 and c2 are integer literals
of type :int, returns |- c1 * c2 = d where d is the canonical integer
literal that is equal to c1 * c2. The literals c1 and c2 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 product of two permitted integer
literals of type :int.
# INT_MUL_CONV `&6 * -- &9`;;
val it : thm = |- &6 * -- &9 = -- &54