Square Root in Transfer Function Reverse Path

Hello,

 as a bit of practice, I'm trying to translate a Simulink system diagram into Matlab.  The Simulink diagram has a "Square Root" block in the feedback path and, for the life of me, I can't think how to implement this in Matlab.  

How can I define a transfer function that performs a square-root of whatever it's passed, in the same way as a transfer function defined as Gi=tf([1],[1 0]) would integrate whatever it's passed?

Any thoughts would be much appreciated,

Thanks :)