Add "const" function, which is the inverse of "nonconst"

src/expr.c

@@ -568,6 +568,10 @@ eval_builtin(expr_node *node, Value *locals, Value *ans, int *nonconst)
         /* All went well; copy the result destructively */
         (*ans) = info.retval;
 
+        /* Special case of const cunction */
+        if (!strcmp(f->name, "const")) {
+            *nonconst = 0;
+        }
         /* Don't allow retval to be destroyed! */
         info.retval.type = ERR_TYPE;
     }

src/funcs.c

@@ -258,6 +258,7 @@ BuiltinFunc Func[] = {
     {   "choose",       2,      NO_MAX, 1,          NULL, FChoose }, /*NEW-STYLE*/
     {   "coerce",       2,      2,      1,          FCoerce, NULL },
     {   "columns",      0,      1,      0,          FColumns, NULL },
+    {   "const",        1,      1,      1,          FNonconst, NULL },
     {   "current",      0,      0,      0,          FCurrent, NULL },
     {   "date",         3,      3,      1,          FDate, NULL },
     {   "datepart",     1,      1,      1,          FDatepart, NULL },