anchor.diff:  
     Solver operator <<(Constraint c) => this..addConstraint(c);
   
  -  _Symbol _getSymbolForVariable(Variable variable) {
  +  _Symbol _symbolForVariable(Variable variable) {
       _Symbol symbol = _vars[variable];
   
       if (symbol != null) {
  
positive.code: 
      expr.terms.forEach((term) {
        if (!_nearZero(term.coefficient)) {
          _Symbol symbol = _getSymbolForVariable(term.variable);
  
          _Row foundRow = _rows[symbol];
  
  
negative.code:       _Row row = _rows[leaving];
  
        if (row != null && row.constant < 0.0) {
          _Symbol entering = _dualEnteringSymbolForRow(row);
  
          if (entering.type == SymbolType.invalid) {
            return Result.internalSolverError;
  