public FunctionInfo registerMacro(String macroName, ExprNodeDesc body,
      List<String> colNames, List<TypeInfo> colTypes) {
    return registerMacro(macroName, body, colNames, colTypes, null);
  }
Registers the UDF class as a built-in function; used for dynamically created UDFs, like
GenericUDFOP*Minus/Plus.
ql/src/java/org/apache/hadoop/hive/ql/exec/Registry.java