"Width equals height" constraint in Interface Builder

Update Xcode 5.1b5

Ctrl+click and drag from a view and release while the pointer is over the view. Select "Aspect Ratio". It will create a constraint where the first and second item is the view.

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Before Xcode 5.1

You can't because the width/height editor lacks the fields to relate to another property or set the ratio:

Therefore, you can't express the following code in Interface Builder:

CGFloat ratio = 1.0;
NSLayoutConstraint *constraint = [NSLayoutConstraint
    constraintWithItem:myView
    attribute:NSLayoutAttributeWidth
    relatedBy:NSLayoutRelationEqual
    toItem:myView
    attribute:NSLayoutAttributeHeight
    multiplier:ratio
    constant:0];
constraint.priority = 1000;
[myView.superview addConstraint:constraint];

Please add a new constraint, aspect ratio to 1:1 on the UI element as in the image.