Fix angle between two bones Is there any way to keep the angle between two bones fixed?

Here is what I am trying to do. I got an armature consisting of three bones. I IK constrained the tail bone to a dummy and when I move the parent bone, the armature should move in a way that the angle between the middle bone and the tail is fixed. I have tried all kinds of tricky stuff, but couldn't get it work.

Here is what I mean (sorry for the annoying logo!).

Basically, I need the angle between the black bone and yellow bone preserved. It is possible whenever the parent bone (blue) and the IK bone (yellow) rotate "more" so to speak. I mean, it's possible mechanically.
If you don't want to substitute both bones with just one:

Select the Yellow bone and under Inverse Kinematics enable all the locks:



Now it's a rigid piece: