code: def _expect_vars(vs=None):
          
          if vs is None:
              return list()
          elif isinstance(vs, Variable):
              return [vs]
          else:
              checked = list()
              # Will raise TypeError if vs is not iterable
              for v in vs:
                  if isinstance(v, Variable):
                      checked.append(v)
                  else:
                      fstr = "expected Variable, got {0.__name__}"
                      raise TypeError(fstr.format(type(v)))
              return checked
docs: Verify the input type and return a list of Variables.