     // MARK: - GraphableEquation
     
     var drawingColor: UIColor = UIColor.black
-    var inputDomain: Range<CGFloat>?
+    var domain: Range<CGFloat>?
     
     // MARK: - Equation
-    
-    func compute(withInterval interval: CGFloat, between x1: CGFloat, and x2: CGFloat) -> [Coordinate]
-    {
-        var coordinates : [Coordinate] = []
-        
-        for x in stride(from: x1, through: x2, by: interval)
-        {
-            coordinates.append(Coordinate(x: x, y:  self.compute(at: x)))
-        }
-        
-        return coordinates
-    }
     
     func compute(at x: CGFloat) -> CGFloat
     {

    // MARK: - GraphableEquation
    
    var drawingColor: UIColor = UIColor.black
    var inputDomain: Range<CGFloat>?
    
    // MARK: - Equation
    
    func compute(withInterval interval: CGFloat, between x1: CGFloat, and x2: CGFloat) -> [Coordinate]
    {
        
        var coordinates : [Coordinate] = []
        
        for x in stride(from: x1, through: x2, by: interval)
        {
            coordinates.append(Coordinate(x: x, y:  (self.compute(at: x))))
        }
        
        return coordinates
    }
    
    func compute(at x: CGFloat) -> CGFloat
    {
        
        return amplitude * cos((self.period * x) - (self.phaseShift/self.period)) + self.verticalShift
    }
}

    let graph = GraphView(withSmallerXBound: -15.0, largerXBound: 15.0, andInterval: 0.5)
    
    let line = Line(slope: 0.5, offset: 0)
//    let sine = Sine()
    
    graph.addEquation(line)
    //    graph.addEquation(sine)
    //: You can add these yourself.
    
    let cosine = Cosine(period: 1.0, amplitude: 1.0, phaseShift: -1.0, verticalShift: 0.0)
    cosine.drawingColor = UIColor(red: 1.0, green: 0.2, blue: 0.2, alpha: 1.0)
    cosine.domain = Range(uncheckedBounds: (lower: 0.0, upper: 8.0))
    
    graph.addEquation(cosine)
    
