1. A method for processing a stream of tuples, each tuple containing one or more attributes, comprising: receiving the stream of tuples to be processed by a plurality of processing elements operating on one or more computer processors; processing, with a first processing method, a group of tuples from the stream of tuples into a grouping window, wherein the first processing method includes: identifying a membership condition, determining whether the membership condition is met by a tuple from the stream of tuples, and including the tuple in the grouping window in response to the membership condition being met, and processing, with a second processing method, a subgroup of tuples from the group of tuples into a subgrouping window, wherein the second processing method includes: identifying a sub-membership condition, determining whether the sub-membership condition is met by a tuple from the group of tuples, and including the tuple in the subgrouping window in response to the sub-membership condition being met.