Add functional attribute to #sizeWordStack (#400)

Author: ttuegel

data.md

@@ -456,9 +456,9 @@ A cons-list is used for the EVM wordstack.
 -   `_in_` determines if a `Int` occurs in a `WordStack`.
 
 ```k
-    syntax Int ::= #sizeWordStack ( WordStack )       [function, smtlib(sizeWordStack)]
-                 | #sizeWordStack ( WordStack , Int ) [function, klabel(sizeWordStackAux), smtlib(sizeWordStackAux)]
- // ----------------------------------------------------------------------------------------------------------------
+    syntax Int ::= #sizeWordStack ( WordStack )       [function, functional, smtlib(sizeWordStack)]
+                 | #sizeWordStack ( WordStack , Int ) [function, functional, klabel(sizeWordStackAux), smtlib(sizeWordStackAux)]
+ // ----------------------------------------------------------------------------------------------------------------------------
     rule #sizeWordStack ( WS ) => #sizeWordStack(WS, 0)
     rule #sizeWordStack ( .WordStack, SIZE ) => SIZE
     rule #sizeWordStack ( W : WS, SIZE )     => #sizeWordStack(WS, SIZE +Int 1)