title: Why does a constraint for the UIViewController.UIView to child UIViewLabel throw error "Invalid pairing of layout attributes"?
body: <p>I am trying to position a label so that the top of the label is 2/3 the way down the UIViewController.  So I wrote this constraint, but it gives me the error below.</p>
  
  <pre><code>NSLayoutConstraint *labelTopConstraint = [NSLayoutConstraint constraintWithItem:self.myLabel
                                           attribute:NSLayoutAttributeTop
                                           relatedBy:NSLayoutRelationGreaterThanOrEqual
                                              toItem:self.view
                                           attribute:NSLayoutAttributeHeight
                                          multiplier:0.66
                                            constant:0];
  </code></pre>
  
  <p>The Error:</p>
  
  <pre><code>*** Terminating app due to uncaught exception 'NSInvalidArgumentException', 
  reason: '*** +[NSLayoutConstraint 
  constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:]: 
  Invalid pairing of layout attributes'
  </code></pre>