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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def p2pkh_input(outpoint, sig, pubkey, sequence=0xFFFFFFFE):
    '''
    OutPoint, hex_string, hex_string, int -> TxIn
    Create a signed legacy TxIn from a p2pkh prevout
    '''
    stack_script = '{sig} {pk}'.format(sig=sig, pk=pubkey)
    stack_script = script_ser.serialize(stack_script)
    return tb.make_legacy_input(outpoint, stack_script, b'', sequence)