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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 parse_instruction(string, location, tokens):
 """Parse instruction. """

    mnemonic_str = ReilMnemonic.from_string(tokens["mnemonic"])

    oprnd1 = tokens["fst_operand"][0]
    oprnd2 = tokens["snd_operand"][0]
    oprnd3 = tokens["trd_operand"][0]

    ins_builder = ReilBuilder()

    return ins_builder.build(mnemonic_str, oprnd1, oprnd2, oprnd3)