Checking the Validity of FDs and Grammars

Before tracing the unification, it is important to check that both the input and the grammar are valid FDs. The following functions perform this checking:

          
(FD-P fd): check that a well-formed FD does not contain inconsistencies
	   (that is, (u fd nil) is not :fail, or in other terms, the FD
           does not contain contradictory features, such as ((a 1) (a 2))).


(GRAMMAR-P grammar): check that a Lisp expression grammar is a well-formed
	   grammar.
          
          

When you suspect that your current input/grammar do not work, check first that they are syntactically valid using these functions.