Add a warning for a such that clause that does not reference the included source

[brynrhodes]

It is valid to write a such that clause that does not reference the included source:

define Valid:
  [Encounter] E
    with [Observation] O such that true

But it is often an indication that something is missing. It would be helpful for the translator to issue a warning in this case:

The such that clause does not reference the included source O. Consider whether this is intended.