Allow definition labels to replace conjunctions

[mn200]

The [foo:] syntax should be able to stand in for conjunctions separating clauses.

This would require the quotation-filter to look for non-comment, non-whitespace characters occurring after conjunctions. If any such occurs, then the next label is a fresh one, and not associated with that conjunction. A free-standing label could turn into a special low-priority (= -1??) infix mapping to conjunction in order to make it clear that these always live at the top-level...