DerivedCondition(formula, *values) - derive condition from value(s)

Usage:

        # 'derived' is re-evaluated whenever x or y change
        derived = DerivedCondition(lambda: x()>=y(), x, y)

A DerivedCondition fires an event equal to formula() whenever any of the supplied values fire, the value of formula() is not equal to its last known value (if any), and the value of formula() is true.

Note that like other events.IConditional implementations, callbacks added to a DerivedCondition will be fired immediately if the current value of formula() is true, and tasks yielding to a true DerivedCondition will also proceed immediately without waiting for a callback.

__init__ (
        self,
        formula,
        *values,
        )