Produces an offspring from two Machines, whose code is a
    combination of the two.

def crossover(m, f):
    """Produces an offspring from two Machines, whose code is a
    combination of the two."""
    i = random.randint(0, len(m.code))
    j = random.randint(0, len(f.code))
    return m.code[:i] + f.code[j:]