anchor.diff:  
       expect(s.addEditVariable(mid.variable, Priority.strong), Result.success);
   
  -    expect(
  -        s.addConstraint((mid * cm(2)).equals(left + right)), Result.success);
  +    expect(s.addConstraint((mid * cm(2)).equals(left + right)), Result.success);
       expect(s.addConstraint(left >= cm(0)), Result.success);
   
       expect(s.suggestValueForVariable(mid.variable, 50), Result.success);
  
positive.code:     expect(s.addConstraint(c), Result.success);
  
      expect(s.addEditVariable(mid.variable, 999), Result.success);
      expect(
          s.addEditVariable(mid.variable, 999), Result.duplicateEditVariable);
      expect(s.removeEditVariable(mid.variable), Result.success);
      expect(s.removeEditVariable(mid.variable), Result.unknownEditVariable);
    });
  
negative.code: 
      final s = Solver();
  
      expect(s.addConstraint((right + left).equals(mid * cm(2))), Result.success);
      expect(s.addConstraint(right - left >= cm(100)), Result.success);
      expect(s.addConstraint(left >= cm(0)), Result.success);
  
  