D.11.2 math library predicates

There is one binary predicate, islessequal, and several unary predicates, isinfinite, isnan, isnormal, isubnormal and iszero.

The predicate islessequal takes a pair of floating point numbers (x,y) as an argument, and returns nil for a false result and (nil,nil) for a true result.

The unary predicates have the obvious interpretations as classification functions, and should probably be used in preference to comparison with constants in case the representations aren't unique.