<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def op_get_opcode_name(op_string):
 """ Get the name of an opcode, given the 'op' byte sequence of the operation. """

    global OPCODE_NAMES

    # special case...
    if op_string == '{}:'.format(NAME_REGISTRATION):
        return 'NAME_RENEWAL'

    op = op_string[0]
    if op not in OPCODE_NAMES:
        raise Exception('No such operation "{}"'.format(op))

    return OPCODE_NAMES[op]