Conversion to perform subtraction on two rational literals of type :real.
The call REAL_RAT_SUB_CONV `c1 - c2` where c1 and c2 are rational
literals of type :real, returns |- c1 - c2 = d where d is the canonical
rational literal that is equal to c1 - c2. The literals c1 and c2 may
be integer literals (&n or -- &n), ratios (&p / &q or -- &p / &q), or
decimals (#DDD.DDDD or #DDD.DDDDeNN). The result d is always put in the
form &p / &q or -- &p / &q with q > 1 and p and q sharing no common
factor, or simply &p or -- &p when that is impossible.
Fails if applied to a term that is not the subtraction function applied to two
permitted rational literals of type :real.