     func insert(symbol: Symbol, coefficient: Double) {
         var coeff = coefficient
 
-        if let existingCoefficient = _cells[symbol] {
+        if let existingCoefficient = cells[symbol] {
             coeff += existingCoefficient
         }
 
         if coeff.isNearZero {
-            _cells[symbol] = nil
+            cells[symbol] = nil
         } else {
-            _cells[symbol] = coeff
+            cells[symbol] = coeff
         }
     }
 

     - returns: The new value of the constant
     */
    func add(_ value: Double) -> Double {
        _constant += value
        return _constant
    }

    /**

            newCells[s] = value
        }

        cells = newCells
    }

    /**
