title: pyhf: POI application using formula
body: <p>I am trying to write a likelihood model in which the POI affects two samples, but while one I have the regular POI*yield, the other I have f(POI)*yield where f is an arbitrary function.</p>
  
  <p>Is there a simple way to implement that in pyhf?</p>
  
  <p>Thanks in advance.</p>