Collated.PrivateSourceval create :
data:('k * 'v) Incr_map_erase_key.t ->
num_filtered_rows:int ->
key_range:'k Collate.Which_range.t ->
rank_range:int Collate.Which_range.t ->
num_before_range:int ->
num_unfiltered_rows:int ->
('k, 'v) t