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Fix compare/3 for variables named _. They are never equal

--- a/builtins.c

+++ b/builtins.c

@@ -247,7 +247,9 @@

 		/* Same type term */

 		switch(t1->tag){

 		case VariableTerm:

-			if(t1->clausenr == t2->clausenr)

+			if(runestrcmp(t1->text, L"_") == 0 && runestrcmp(t2->text, L"_") == 0)

+				result = 1; /* Special case since _ and _ are always different */

+			else if(t1->clausenr == t2->clausenr)

 				result = runestrcmp(t1->text, t2->text);

 			else

 				result = Compare(t1->clausenr, t2->clausenr);