1. A computer program product for processing a stream of tuples, each tuple containing one or more attributes, the computer program product comprising a computer readable storage medium having program code embodied therewith, the program code comprising computer readable program code configured to: process the attributes of the stream of tuples using stream operators operating on one or more computer processors and corresponding one or more processing elements; process, with a first processing method, a group of tuples from the stream of tuples into a grouping window, wherein the first processing method is configured to: identify a membership condition, determine whether the membership condition is met by a tuple from the stream of tuples, and include the tuple in the grouping window in response to the membership condition being met; and process, with a second processing method, a subgroup of tuples from the group of tuples into a subgrouping window, wherein the second processing method is configured to: identify a sub-membership condition. determine whether the sub-membership condition is met by a tuple from the group of tuples, and include the tuple in the subgrouping window in response to the sub-membership condition being met.