That should be even 0 = true even n = odd n-1 odd 0 = false odd n = even n-1 I fed a C version of this (with unsigned n to keep the nasal daemons at

That should be even 0 = true even n = odd n-1 odd 0 = false odd n = even n-1 I fed a C version of this (with unsigned n to keep the nasal daemons at bay) to clang and observed that it somehow manages to see through the mutual recursion, generating code that doesn't recurse or loop.