The question mark is the standard Scheme way of writing a predicate, for example see https://www-sop.inria.fr/indes/fp/Bigloo/doc/r5rs-9.html#Numerical-operatio

The question mark is the standard Scheme way of writing a predicate, for example see https://www-sop.inria.fr/indes/fp/Bigloo/doc/r5rs-9.html#Numerical-operations https://www-sop.inria.fr/indes/fp/Bigloo/doc/r5rs-9.html#Num...