Please try the following:
Leads ["1", "2", "3", "4", "5", "6", "7", "8", "9", "10"]
request 1 "id:0:ts12345:os:0:rc:0" set batchSize to 5
returns: ["1", "2", "3", "4", "5"] and a new stream position of "id:5:ts:12345:os:5:rc:5"
Delete lead "4" which has already been retrieved
request 2 "id:5:ts:12345:os:5:rc:5"
returns ["7", "8", "9", "10"]
Lead "6" was not returned, but it has not been deleted.