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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 _assemble_autophosphorylation(stmt):
 """Assemble Autophosphorylation statements into text."""

    enz_str = _assemble_agent_str(stmt.enz)
    stmt_str = enz_str + ' phosphorylates itself'
    if stmt.residue is not None:
        if stmt.position is None:
            mod_str = 'on ' + ist.amino_acids[stmt.residue]['full_name']
        else:
            mod_str = 'on ' + stmt.residue + stmt.position
    else:
        mod_str = ''
    stmt_str += ' ' + mod_str
    return _make_sentence(stmt_str)