DerivedValue(formula, *values) - a value derived from other values

Usage:

        # 'derived' changes whenever x or y change
        derived = DerivedValue(lambda: x()+y(), x, y)

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

__call__ ( self )

__init__ (
        self,
        formula,
        *values,
        )

_set (
        self,
        source,
        event,
        )