Add [IF] .. [THEN]

Will be useful as kinds of "#ifdef"s during xcomp.

blk/042

@@ -7,10 +7,10 @@ context, branching doesn't work.
 
 f IF A ELSE B THEN: if f is true, execute A, if false, execute
 B. ELSE is optional.
+[IF] .. [THEN]: Meta-IF. Works outside definitions. No [ELSE].
 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. x is dropped
+
-in case of an OF match, *but it is kept if it reaches C*. You
+                                                        (cont.)
-have to consume it to avoid PSP leak.                   (cont.)

blk/043

@@ -1,4 +1,8 @@
-(cont.)
+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. 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.           
+
 (br)      --   Branches by the number specified in the 2
                following bytes. Can be negative.
 (?br)     f -- Branch if f is false.
@@ -9,8 +13,3 @@
 ABORT     --   Resets PS and RS and returns to interpreter.
 ABORT" x" --   *I* Compiles a ." followed by a ABORT.
 EXECUTE   a -- Execute wordref at addr a
-INTERPRET --   Get a line from stdin, compile it in tmp memory,
-               then execute the compiled contents.
-LEAVE     --   In a DO..LOOP, exit at the next LOOP call.
-QUIT      --   Return to interpreter prompt immediately
-EXIT!     --   Exit current INTERPRET loop.

blk/044

@@ -0,0 +1,5 @@
+INTERPRET --   Get a line from stdin, compile it in tmp memory,
+               then execute the compiled contents.
+LEAVE     --   In a DO..LOOP, exit at the next LOOP call.
+QUIT      --   Return to interpreter prompt immediately
+EXIT!     --   Exit current INTERPRET loop.

blk/426

@@ -6,3 +6,7 @@
     H@ 2-          ( push a. -2 for allot offset )
 ; IMMEDIATE
 
+: [IF]
+    IF EXIT THEN
+    LIT< [THEN] BEGIN DUP WORD S= UNTIL DROP ;
+: [THEN] ;