Variant

- A variant now contains a source reference, which indicates where it came
  from.  This is used to determine whether 'H' is a priority or just a word.
Filter

- When a filter resolves an identifier through a DOM reference, it records
  the source. In other words, when 'priority' is resolved to 'H', the source
  reference ('priority') is stored in the variant. This is because different
  data attributes behave differently when compared.
Variant

- Added specialized handling for 'priority' attributes when compared using the
  <, <=, > or >= operators.