saxon:stream($input as item()*) ==>item()*
Conceptually, this function returns a copy of its input. The intent, however, is to evaluate the supplied argument in "streaming mode", which allows an input document to be processed without building a tree represention of the whole document in memory. This allows much larger documents to be processed using Saxon than would otherwise be the case.
For further details see Streaming of Large Documents.
The saxon:stream() function is available only with Saxon-EE.
