code: def op_get_mutate_fields( op_name ):
      
      global MUTATE_FIELDS
  
      if op_name not in MUTATE_FIELDS.keys():
          raise Exception("No such operation '%s'" % op_name)
  
      fields = MUTATE_FIELDS[op_name][:]
      return fields
docs: Get the names of the fields that will change
      when this operation gets applied to a record.