Mutates code.

def mutate(self):
        """Mutates code."""
        # Choose a random position
        if len(self.code) == 0:
            return

        index = random.randint(0, len(self.code)-1)
        mutation_type = random.random()

        if mutation_type < 0.5:
            # Change
            self.code[index] = self.new().randomize().code[0]
        elif mutation_type < 0.75:
            # Deletion
            del self.code[index]
        else:
            # Insertion
            self.code.insert(index, self.new().randomize().code[0])