[(Unsuccessful) messing with backtracking, another test case for split.
gergo@complang.tuwien.ac.at**20090126102518] {
hunk ./qsort-example.fs 28
+  \ ." split clause 1" cr
hunk ./qsort-example.fs 33
+  \ prolog-success @ if ." matched fact 1" cr endif
hunk ./qsort-example.fs 42
+  \ ." split clause 2" cr
hunk ./qsort-example.fs 49
+  \ prolog-success @ if ." matched head 2" cr endif
hunk ./qsort-example.fs 61
+  \ ." split clause 3" cr
hunk ./qsort-example.fs 71
+  \ prolog-success @ if ." matched head 3" cr endif
hunk ./qsort-example.fs 90
+  \ ." split clause 4" cr
hunk ./qsort-example.fs 100
+  \ prolog-success @ if ." matched head 4" cr else
+  \     ." failing goal: split("
+  \         a1 @ unparse ." , "
+  \         a2 @ unparse ." , "
+  \         a3 @ unparse ." , "
+  \         a4 @ unparse ." )" cr
+  \ endif
hunk ./qsort-example.fs 130
+4 create-integer create four ,
+5 create-integer create five ,
hunk ./qsort-example.fs 173
+
+." :- split(Xs, 3, [1,2], [4,5])" cr
+create-unbound create xs ,
+:noname
+    ." Xs = " xs @ unparse cr
+    ; is print-solutions
+one @ two @ create-nil create-list create-list create lowers ,
+four @ five @ create-nil create-list create-list create uppers ,
+xs @ a1 !
+pivot-three @ a2 !
+lowers @ a3 !
+uppers @ a4 !
+' split prolog-shell cr
hunk ./wam.fs 640
+  \ FIXME: Alternative solutions for split(Xs, 3, [1,2], [4,5]) are not
+  \ found using this code.
hunk ./wam.fs 648
-            pop-choice-point
+            global-state cp-b @ stack > if
+                pop-choice-point
+            endif
hunk ./wam.fs 653
-                alternative pl-call
+                global-state cp-bp @ pl-call
hunk ./wam.fs 655
-                alternative pl-execute
+                global-state cp-bp @ pl-execute
hunk ./wam.fs 659
+            global-state cp-b @ stack > if
+              \ ." cp-b @ = " global-state cp-b @ .
+              \ ." , stack = " stack .
+              \ ." , trying to pop choice point to retry" cr
+                pop-choice-point
+              \ -1 prolog-success !
+                recurse
+            endif
hunk ./wam.fs 671
+:noname ( flag -- )
+    begin
+      \ Find a choice point with a non-fail alternative, if any.
+        fail global-state cp-bp @ =
+        global-state cp-b @ stack > and while
+            pop-choice-point
+    repeat
+    global-state cp-bp @ { alternative }
+    fail alternative <> if
+      \ Found one, retry.
+        -1 prolog-success !
+        ( flag )
+        0= if
+            alternative pl-call
+        else
+            alternative pl-execute
+        endif
+    endif
+    ; \ is try-alternatives
+
hunk ./wam.fs 875
+          \ ." global-s = " global-s . cr
+          \ ." global-s @ = " global-s @ . cr
+          \ ." global-s @ @ = " global-s @ @ . cr
+          \ ." global-s @ @ @ = " global-s @ @ @ . cr
+          \ ." global-s @ @ = " global-s @ @ unparse cr
}