These routines perform RUSSELL signature deduction and checking.


Invariants and general observations:

1.   Signatures never contain implied selections,
   applications, V calls, or unresolved id references.
   This invariant is established (by fiat) in the previous pass.
     One consequence of this is that subst should never be asked
   to substitute an incompletely specified expression inside
   an arbitrary signature. (See 3 below however.)
     Checksigs makes sure that selections in signatures are unambiguous,
   independent of signature information.

2.   If findsig is called on a local anonymous type identifier
   it sets the signature field to an appropriate value, but
   not the sig_done field.  This is necessary since such identifier
   nodes are shared eventhough they may have different signatures
   in different contexts.  Thus the signature is recomputed the
   next time the same node is encountered.  Note that the signature
   of all such nodes is constant during a single top level call
   to findsig.  Thus resetting the signature field of such a node
   is guaranteed not to clobber an old value which is still needed.

3.  Findsig always determines the signatures of all subexpressions,
   whether they are needed immediately or not.  Findsig also inserts
   all omited selections, V calls, and constant applications.  Thus
   it is generally safe to ask subst to substitute an expression into a
   signature, provided findsig has been called on the subexpression,
   as evidenced by the sig_done field.
    The only problem with this is that some signatures can, and in order
   to avoid unnecessary circularities should be, determined before
   the signatures of all the subexpressions are known.  In this case
   such an expression is put on the dontsubst list, and only an
   attempt to actually substitute it will cause a circularity
   to be detected.
    Note that the dontsubst list can be cleared after a top level call
   of findsig is complete since checksigs works bottom up.  Thus 
   a top level call of findsig will only be made if the signatures
   of all subexpressions are already known.
    Missing constant applications, if they cannot be filled in the first
   time, are filled in by the appropriate top level call to findsig.

4.  The id_last_definition fields of identifiers may occasionally point
   to the original declaration node of the identifier, and not to
   subsequently made copies.  
    For local type identifiers and parameters, it must be possible to
   check easily whether an identifier is the one declared in a given
   parameter node or type signature node.  Preorder numbers are copied
   with the rest of a node, so they can be used to determine this.
    (This whole scheme is probably no longer needed, but it would take
   some time to get rid of its remaining uses.)