title: How to change Auto Layout constraints after they are set when using constraintEqualToAnchor()?
body: <p>I try to set up a view with AutoLayout constraints by using <code>constraintEqualToAnchor()</code>:</p>
  
  <pre><code>override func viewDidLoad() {
      super.viewDidLoad()
  
      let myView = UIView()
      myView.backgroundColor = UIColor.orangeColor()
      myView.translatesAutoresizingMaskIntoConstraints = false
      view.addSubview(myView)
  
      myView.leftAnchor.constraintEqualToAnchor(view.leftAnchor).active = true
      myView.trailingAnchor.constraintEqualToAnchor(view.trailingAnchor).active = true
      myView.topAnchor.constraintEqualToAnchor(view.topAnchor).active = true
      myView.bottomAnchor.constraintEqualToAnchor(view.bottomAnchor).active = true
  
      /******************************************/
      /* I try to change one of the constraints */
      /******************************************/
      myView.leftAnchor.constraintEqualToAnchor(view.rightAnchor, constant: -100).active = true  
  }
  </code></pre>
  
  <p>In the last line of code, I try to change one of the constraints. I thought it would work but it gives some error in the console log</p>
  
  <pre><code>"&lt;NSLayoutConstraint:0x7fb53a5180d0 H:|-(0)-[UIView:0x7fb53a5190b0](LTR)   (Names: '|':UIView:0x7fb53a519240 )&gt;",
  "&lt;NSLayoutConstraint:0x7fb53a51f660 H:[UIView:0x7fb53a519240]-(-100)-[UIView:0x7fb53a5190b0](LTR)&gt;",
  "&lt;NSLayoutConstraint:0x7fb53a711ee0 'UIView-Encapsulated-Layout-Width' H:[UIView:0x7fb53a519240(414)]&gt;"
  </code></pre>
  
  <p>When using <code>constraintEqualToAnchor()?</code>, what's the right way to change the constraint later after I have set them? </p>