Comment 2 for bug 396095

Possible algorithm for derived variables of user defined class:

Instance must have an attribute:

data _dt: Pointer

Function to return/create differential:

func __diff__(self):
    if self._dt.is_set():
        return self._dt.get()
    else:
        data m: MyClass
        associate_state_diff(self.x, m.x)
        self._dt = m
        return m