double x = x + x

-- double (1 + 2) --> (1 + 2) + (1 + 2) --> 3 + (1 + 2) --> 3 + 3 --> 6

-- Merge (infinite!) lists of increasing numbers in an ordered way:
ordMerge (x:xs) (y:ys) | x == y = x : ordMerge xs ys
                       | x <  y = x : ordMerge xs (y:ys)
                       | x >  y = y : ordMerge (x:xs) ys

ordMerge3 xs ys zs = ordMerge xs (ordMerge ys zs)

hamming = 1 : ordMerge3 (map (*2) hamming)
                        (map (*3) hamming)
                        (map (*5) hamming)

nats = 0 : map (+1) nats