title: PyMC observed data for a sum of random variables
body: <p>I'm trying to infer models parameters with PyMC. In particular the observed data is modeled as a sum of two different random variables: a negative binomial and a poisson.</p>
  
  <p>In PyMC, an algebraic composition of random variables is described by a "deterministic" object. Is it possible to assign the observed data to this deterministic object?</p>
  
  <p>If not possible, we still know that the PDF of the sum is the convolution the PDF of the components. Is there any trick to compute this convolution efficiently?</p>