anchor.diff:      return new _Symbol(SymbolType.invalid, 0);
     }
   
  -  _Pair<_Symbol, _Row> _getLeavingRowForEnteringSymbol(_Symbol entering) {
  +  _Pair<_Symbol, _Row> _leavingRowForEnteringSymbol(_Symbol entering) {
       double ratio = double.MAX_FINITE;
       _Pair<_Symbol, _Row> result = new _Pair(null, null);
   
  
positive.code: 
    Solver operator <<(Constraint c) => this..addConstraint(c);
  
    _Symbol _getSymbolForVariable(Variable variable) {
      _Symbol symbol = _vars[variable];
  
      if (symbol != null) {
  
negative.code:       _rows.remove(tag.marker);
      } else {
        _Pair<_Symbol, _Row> rowPair =
            _leavingRowPairForMarkerSymbol(tag.marker);
  
        if (rowPair == null) {
          return Result.internalSolverError;
  