Constraints to center and object between two objects

My design requires that a button is centered between two other objects. One of the objects is in the vertical center of the view (green). The other object is some distance from the bottom edge (green). Now the task is to center the third object (red) between the other two. I am using xcode6's new constraints and my view is in wRegular hRegular mode. This would be easy with code, but I am trying to use the storyboard to accomplish this.

There are a number of approaches:

In iOS 9, the easiest would be to define a vertical UIStackedView with a distribution of "equal spacing" and then addArrangedSubview the three circular subviews
Another option in iOS 9 would be to create two UILayoutGuide (which would represent the two question marks in your image), add them to the shared superview with addLayoutGuide and define them to be the same size as each other. The corresponding VFL might look like:
```
"V:|[greenView1(==100)]-[layoutGuide1]-[redView(==50)]-[layoutGuide2(==layoutGuide1)]-[greenView2(==greenView1)]|"
```
In earlier iOS versions, rather than using UILayoutGuide, you could just create two "space" views (UIView with clear background so they're not visible) then define a constraint such that their heights are identical, and then define vertical spacing constraints between the five views (the three circles and the two spacer views) with a constant of zero.

It might look like:

I've made those "spacer" views visible, to illustrate the idea, but obviously you'd set them to be transparent so you can't see them in the UI.

This is logically equivalent to the UILayoutGuide approach in iOS 9, except that the UIView "spacer" views just carry a little more overhead than a UILayoutGuide. But in iOS versions before 9, this is the common approach to this problem.