Note: I've removed a portion of this description because it was separated into a new ticket (DFDL-412)

asserts/discriminators with testKind='pattern' the pattern must match a prefix of the current textual data stream.
This match attempt is done before the element is parsed.

(Our combinator which evaluates elements with their associated DFDL statement annotations - asserts, setVariable, etc. - this must be changed to evaluate asserts/discriminators with testKind='pattern' first.)