[Backtracking works! At least when executing concat-tests.
Gergö Barany <gergo@complang.tuwien.ac.at>**20090120231406] {
hunk ./wam.fs 274
+  \ ." *** pushing cp" cr
hunk ./wam.fs 279
-    b stack-max >= if
-        ." stack overflow" exception throw
+    new-b stack-max >= if
+        s" Prolog choicepoint stack overflow" exception throw
hunk ./wam.fs 289
+  \ ." *** popping cp" cr
hunk ./wam.fs 291
-    \ Trail variables!
+    global-state cp-b @ { prev-cp }
+    \ Underflow check.
+    prev-cp stack <= if
+        s" Prolog choicepoint stack underflow" exception throw
+    endif
+    \ Trail variables: Make all variables between the top-of-trail and the
+    \ previous top-of-trail unbound.
+    global-state cp-tr { trailptr }
+  \ ." trailptr: " trailptr @ . cr
+    prev-cp cp-tr @ { top-of-trail }
+  \ ." top-of-trail: " top-of-trail . cr
+    begin
+        trailptr @ top-of-trail > while
+            trailptr @ cell - trailptr !
+          \ ." unbinding " trailptr @ . ." / " trailptr @ @ . cr
+          \ trailptr @ @ tag . cr
+            trailptr @ @ unbind
+            \ unbind leaves the pointer to the variable (now unbound) on the
+            \ stack, but we don't need it here anymore.
+            drop
+    repeat
+    \ Copy old choicepoint into current state.
+    prev-cp global-state choicepoint% %size move
hunk ./wam.fs 317
+  \ ." set-alternative " dup . cr
hunk ./wam.fs 323
+  \ ." trailing " dup . ." : "
hunk ./wam.fs 325
+  \ global-state cp-tr @ . ." -> "
hunk ./wam.fs 327
+  \ global-state cp-tr @ . cr
hunk ./wam.fs 344
+fail global-state cp-bp !
hunk ./wam.fs 573
+    global-state cp-bp @ { alternative }
+    alternative fail <> if
+      \ ." trying alternative clause " alternative . cr
+      \ The alternative pointer is not equal to fail. That is, there is an
+      \ alternative; execute it! I think this means we must pop the choice
+      \ point here.
+      pop-choice-point
+      alternative recurse
+    endif
hunk ./wam.fs 588
+  \ ." proceed" cr
hunk ./wam.fs 597
+      \ TODO: Ask whether to search for additional solutions?
hunk ./wam.fs 762
+  \ ." try_me_else" cr
hunk ./wam.fs 766
+\ wam-instr: trust_me_else ( xt -- )
+\     ." trust_me_else" cr
+\     pop-choice-point
+\     \ In practice, this will always be called with the fail xt. Thus setting
+\     \ the alternative is absolutely unnecessary. But let's do it anyway.
+\     set-alternative
+\     push-choice-point
+\     ;wam-instr
hunk ./wam.fs 775
-    pop-choice-point
-    \ In practice, this will always be called with the fail xt. Thus setting
-    \ the alternative is absolutely unnecessary. But let's do it anyway.
+  \ ." trust_me_else new!" cr
hunk ./wam.fs 906
+  \ ." C1a" cr
hunk ./wam.fs 911
+  \ ." C1" cr
hunk ./wam.fs 919
+  \ ." C2a" cr
hunk ./wam.fs 924
+  \ ." C2" cr
hunk ./wam.fs 1024
-        ." A = " global-a-var @ unparse cr
+        ." A = " global-a-var @ unparse ." ," cr
hunk ./wam.fs 1027
-        ." B = " global-b-var @ unparse cr
+        ." B = " global-b-var @ unparse ."  ;" cr cr
hunk ./wam.fs 1043
-    ." ;" cr
-    print-solutions
+  \ ." ;" cr
+  \ print-solutions
hunk ./wam.fs 1054
+  \ ." C1:  " C1 @ . cr
+  \ ." C1a: " C1a @ . cr
+  \ ." C2:  " C2 @ . cr
+  \ ." C2a: " C2a @ . cr
}