Preliminary code review for domain CHECK constraints patch: add documentation,

make VALUE a non-reserved word again, use less invasive method of passing
ConstraintTestValue into transformExpr, fix problems with nested constraint
testing, do correct thing with NULL result from a constraint expression,
remove memory leak.  Domain checks still need much more work if we are going
to allow ALTER DOMAIN, however.

src/include/nodes/parsenodes.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: parsenodes.h,v 1.222 2002/12/12 15:49:40 tgl Exp $
+ * $Id: parsenodes.h,v 1.223 2002/12/12 20:35:16 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -275,15 +275,6 @@ typedef struct InsertDefault
 	NodeTag		type;
 } InsertDefault;
 
-/*
- * Empty node used as raw-parse-tree representation of VALUE keyword
- * for domain check constraints.
- */
-typedef struct DomainConstraintValue
-{
-	NodeTag		type;
-} DomainConstraintValue;
-
 /*
  * SortGroupBy - for ORDER BY clause
  */