* Goal-directed proof now enforces strict proof irrelevance wrt. sort
hypotheses. Sorts required in the course of reasoning need to be
covered by the constraints in the initial statement, completed by the
type instance information of the background theory. Non-trivial sort
hypotheses, which rarely occur in practice, may be specified via
vacuous propositions of the form SORT_CONSTRAIN('a::c). For example:
lemma assumes "SORT_CONSTRAINT('a::empty)" shows False ...
The result contains an implicit sort hypotheses as before --
SORT_CONSTRAINT premises are eliminated as part of the canonical
rule normalization.
