      Reverse the sign of the constant and all cells in the row.
      */
     func reverseSign() {
-        _constant = -_constant
+        constant = -constant
 
         var newCells = OrderedDictionary<Symbol, Double>()
 

    func solveFor(_ symbol: Symbol) {
        let coeff = -1.0 / cells[symbol]!
        cells[symbol] = nil
        _constant *= coeff

        var newCells = OrderedDictionary<Symbol, Double>()


            newCells[symbol] = value
        }

        cells = newCells
    }

    /**
