title: Why is NSLayoutAttributeCenterX an 'Invalid pairing' with NSLayoutAttributeWidth
body: <p>I am trying to have the auto layout manager adjust the center point of a view, based on the width of the super view.  I don't understand why that is an 'Invalid Pairing' of the attributes (as told by the crash and NSInvalidArgumentException)</p>
  
  <pre><code>UIView *ac;
  NSLayoutConstraint *cXloc = [NSLayoutConstraint constraintWithItem:ac 
                                                           attribute:NSLayoutAttributeCenterX 
                                                           relatedBy:NSLayoutRelationEqual 
                                                              toItem:ac.superview 
                                                           attribute:NSLayoutAttributeWidth 
                                                          multiplier:.1 
                                                            constant:x*ac.superview.frame.size.width*.2];
  [ac.superview addConstraint:cXloc];
  </code></pre>
  
  <p>Could someone explain why this is an 'Invalid paring' and how I should approach this?
  Thanks</p>