code: def get_difference_model(self, category):
          
          name = self.diffname(self.ivarmap[category])
          return self.modelstr.get(name)
docs: Get the equation corresponding to a variation wrt category. For example
          if::
  
              modelstr = {
                  'full' :'H(I) + B',
                  'dH' : 'dH(I)',
                  'dI' : 'H(dI)',
                  'dB' : 'dB'
              }
  
              varmap = {'H': 'psf', 'I': 'obj', 'B': 'bkg'}
  
          then ``get_difference_model('obj') == modelstr['dI'] == 'H(dI)'``