Prevents chunks from travelling down the stream until the predicate returns true.
--0--1--2--3--4--5--6--1--2--3--4--after(x => x > 4)-----------------5--6--1--2--3--4-- Copy
--0--1--2--3--4--5--6--1--2--3--4--after(x => x > 4)-----------------5--6--1--2--3--4--
Prevents chunks from travelling down the stream until the predicate returns true.