def is_16bit_oper_register(x):
    """ Returns whether the given string x is a 16 bit register. These are any 16 bit register
    which belongs to the normal (documented) Z80 instruction set as operands.
    """
    return x.lower() in {'af', "af'", 'bc', 'de', 'hl', 'ix', 'iy', 'sp'}
