Conversion to produce absolute value of an integer literal of type :int.
The call INT_ABS_CONV `abs c`, where c is an integer literal of type
:int, returns the theorem |- abs c = d where d is the canonical integer
literal that is equal to c's absolute value. 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.
# INT_ABS_CONV `abs(-- &42)`;;
val it : thm = |- abs (-- &42) = &42