fix dif:vars_remove_goal/2 (#2175), untabify forms.rs to appease format check

src/lib/dif.pl

@@ -33,11 +33,13 @@ remove_goal([G0|G0s], Goal0, Goals) :-
 
 vars_remove_goal([], _).
 vars_remove_goal([Var|Vars], Goal0) :-
-    get_atts(Var, +dif(Goals0)),
+    (  get_atts(Var, +dif(Goals0)) ->
        remove_goal(Goals0, Goal0, Goals),
        (   Goals = [] ->
            put_atts(Var, -dif(_))
        ;   put_atts(Var, +dif(Goals))
+       )
+    ;  true
     ),
     vars_remove_goal(Vars, Goal0).
 

src/tests/dif.pl

@@ -207,6 +207,13 @@ test("scryer-prolog#2056",(
     \+ E=[]
 )).
 
+% https://github.com/mthom/scryer-prolog/issues/2175
+test("scryer-prolog#2175",(
+    dif(A,B),
+    A=_C*[],
+    A=[]*D*B,D=[]
+)).
+
 main :-
     findall(test(Name, Goal), test(Name, Goal), Tests),
     run_tests(Tests, Failed),