def SUM(src_column):
  """
  Builtin sum aggregator for groupby.

  Example: Get the sum of the rating column for each user. If
  src_column is of array type, if array's do not match in length a NoneType is
  returned in the destination column.

  >>> sf.groupby("user",
  ...            {'rating_sum':tc.aggregate.SUM('rating')})

  """
  return ("__builtin__sum__", [src_column])