Give CASE's default branch access to its input value

ref #97

blk/042

@@ -1,9 +1,9 @@
 Flow
 
-Note about flow words: flow words can only be used in
-definitions. In the INTERPRET loop, they don't have the desired
-effect because each word from the input stream is executed
-immediately. In this context, branching doesn't work.
+Note that flow words can only be used in definitions. In the
+INTERPRET loop, they don't have the desired effect because each
+word from the input stream is executed immediately. In this
+context, branching doesn't work.
 
 f IF A ELSE B THEN: if f is true, execute A, if false, execute
 B. ELSE is optional.
@@ -11,6 +11,6 @@ BEGIN .. f UNTIL: if f is false, branch to BEGIN.
 BEGIN .. AGAIN: Always branch to BEGIN.
 x y DO .. LOOP: LOOP increments y. if y != x, branch to DO.
 x CASE y OF A ENDOF z OF B ENDOF C ENDCASE: If x == y, execute
-A, if x == z, execute B. Otherwise, execute C.
-
-                                                        (cont.)
+A, if x == z, execute B. Otherwise, execute C. x is dropped
+in case of an OF match, *but it is kept if it reaches C*. You
+have to consume it to avoid PSP leak.                   (cont.)

blk/427

@@ -1,10 +1,10 @@
 ( During a CASE, the stack grows by 1 at each ENDOF so that
   we can fill all those ENDOF branching addrs. So that we
   know when to stop, we put a 0 on PSP. That's our stopgap. )
-: CASE 0 COMPILE >R ; IMMEDIATE
+: CASE 0 ; IMMEDIATE
 : OF
-    COMPILE I COMPILE =
-    [COMPILE] IF
+    COMPILE OVER COMPILE =
+    [COMPILE] IF COMPILE DROP
 ; IMMEDIATE
 : ENDOF [COMPILE] ELSE ; IMMEDIATE
 

blk/428

@@ -1,11 +1,9 @@
 ( At this point, we have something like "0 e1 e2 e3 val". We
-  want top drop val, and then call THEN as long as we don't
+  want to drop val, and then call THEN as long as we don't
   hit 0. )
 : ENDCASE
     BEGIN
-        DUP NOT IF
-            DROP COMPILE R> COMPILE DROP EXIT
-        THEN
+        DUP NOT IF DROP EXIT THEN
         [COMPILE] THEN
     AGAIN
 ; IMMEDIATE

tests/forth/test_flow.fs

@@ -3,11 +3,12 @@
         'X' OF 42 ENDOF
         0x12 OF 43 ENDOF
         255 OF 44 ENDOF
-        45
+        1+
     ENDCASE
 ;
 
 'X' foo 42 #eq
 0x12 foo 43 #eq
 255 foo 44 #eq
-254 foo 45 #eq
+254 foo 255 #eq
+'S S0 #eq