reader[make_pair("sqrt", 1)] = sqrt_reader;
reader[make_pair("pow", 2)] = pow_reader;
reader[make_pair("power", 2)] = power_reader;
reader[make_pair("sqrt", 1)] = sqrt_reader;
reader[make_pair("pow", 2)] = pow_reader;
reader[make_pair("power", 2)] = power_reader;