Treat the absence of a flag in a pattern the same way as a false flag.

Fixes a case where multiple patterns added with the same addAssertion call did not have the same set of flags. Regression test enclosed.
Implemented 'not' variants of >=, =, <, <=, positive, and negative assertions so they can be delegated to with [not].
Implemented 'to be close to' assertion.

Fixes #42.