code: def _get_hook(self, shader, name):
          
          assert name in ('pre', 'post')
          key = (shader, name)
          if key in self._hooks:
              return self._hooks[key]
          hook = StatementList()
          if shader == 'vert':
              self.view_program.vert[name] = hook
          elif shader == 'frag':
              self.view_program.frag[name] = hook
          self._hooks[key] = hook
          return hook
docs: Return a FunctionChain that Filters may use to modify the program.
  
          *shader* should be "frag" or "vert"
          *name* should be "pre" or "post"