h : N -> N
h(0)    = 1              -- matches 0
h(2k+1) = h(k)           -- matches any natural of the form 2k+1 for a natural number k
h(2k+2) = h(k+1) + h(k)  -- matches any natural of the form 2k+2

isHalf : Q -> Bool
isHalf(s) = {? true when s is _ / 2,  -- matches fractions with denominator 2
               false otherwise ?}