Find command to provide feedback when an identifier does not exist
In the find command any identifier not known in the current machine is implicitly existentially quantified. This is a bit dangerous here, as a typo will not result in user feedback. It is probably a good idea to required closed formulas here, i.e., not perform implicit quantification. This may require work on the Prolog side.
(In other contexts, where the solution is given back to the user, implicit quantification is very useful; e.g. for the :solve command).