anchor.diff:  
       expr.terms.forEach((term) {
         if (!_nearZero(term.coefficient)) {
  -        _Symbol symbol = _getSymbolForVariable(term.variable);
  +        _Symbol symbol = _symbolForVariable(term.variable);
   
           _Row foundRow = _rows[symbol];
   
  
positive.code:       _rows.remove(tag.marker);
      } else {
        _Pair<_Symbol, _Row> rowPair =
            _getLeavingRowPairForMarkerSymbol(tag.marker);
  
        if (rowPair == null) {
          return Result.internalSolverError;
  
negative.code: 
    Solver operator <<(Constraint c) => this..addConstraint(c);
  
    _Symbol _symbolForVariable(Variable variable) {
      _Symbol symbol = _vars[variable];
  
      if (symbol != null) {
  