'Send a change event notification if the property is set to a
value is not equal to ``old``.
Args:
obj (HasProps)
The object the property is being set on.
old (obj) :
The previous value of the property to compare
Raises:
NotImplementedError
**Subclasses must implement this to serve their specific needs.**'

def trigger_if_changed(self, obj, old):

    raise NotImplementedError('Implement    trigger_if_changed()')
