title: Statsmodels formula API (patsy): How to exclude a subset of interaction components?
body: <p>I'm building a WLS (<code>statsmodels.formula.api.wls</code>) model using the statsmodels formulas API (from patsy) and I'm using interactions between factors. Some of these are predictive whereas others are not. Is there a way to include only a subset of the interactions in the model without resorting to building a design matrix by hand?</p>
  
  <p>Alternatively, is there a way to constrain the estimated coefficients of a subset of the model variables to be equal to zero? </p>