public GenericUDF negative() {
    throw new UnsupportedOperationException("Negative function doesn't exist for " + getFuncName());
  }
Gets the negative function of the current one. E.g., GenericUDFOPNotEqual for
GenericUDFOPEqual, or GenericUDFOPNull for GenericUDFOPNotNull.
@return Negative function
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDF.java