Use proper apostrophe marks in comment

Previous ones were copied from a pdf of a paper, and they were breaking CLISP on Windows
Bind seen-positions hashtable to a context

This is so that it doesn't invalidate when used with parse-sequence,
which will continue parsing to obtain additional results after leaving
parse-sequence dynamic scope as required.

Also make type declaration in note-position use long-form to placate
ECL compiler (fixes #11).