Aggregate constructor @param gbfield the 0-based index of the group-by field in the tuple, or NO_GROUPING if there is no grouping @param gbfieldtype the type of the group by field (e.g., Type.INT_TYPE), or null if there is no grouping @param afield the 0-based index of the aggregate field in the tup
(int gbfield, Type gbfieldtype, int afield, Op what)
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nothing calls this directly
no outgoing calls
no test coverage detected