[WWcvs] pg: Reduce -0 to 0.

[dpvc v. a. <we...@ma...>]

Log Message:
-----------
Reduce -0 to 0.

Modified Files:
--------------
    pg/lib/Parser/UOP:
        minus.pm

Revision Data
-------------
Index: minus.pm
===================================================================
RCS file: /webwork/cvs/system/pg/lib/Parser/UOP/minus.pm,v
retrieving revision 1.5
retrieving revision 1.6
diff -Llib/Parser/UOP/minus.pm -Llib/Parser/UOP/minus.pm -u -r1.5 -r1.6
--- lib/Parser/UOP/minus.pm
+++ lib/Parser/UOP/minus.pm
@@ -29,11 +29,13 @@
 sub _reduce {
   my $self = shift; my $op = $self->{op};
   my $reduce = $self->{equation}{context}{reduction};
-  $self = $op->{op} if $op->isNeg && $reduce->{'-(-x)'};
+  return $op->{op} if $op->isNeg && $reduce->{'-(-x)'};
+  return $op if $op->{isZero} && $reduce->{'-0'};
   return $self;
 }
 
 $Parser::reduce->{'-(-x)'} = 1;
+$Parser::reduce->{'-0'} = 1;
 
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