In most cases you don't have to call this method. enable() is doing this
for you. But if the mock is defined after the first call in the
tested class, the tested class doesn't resolve to the mock. This is
documented in Bug #68541. You therefore have to define the namespaced
function before the first call. Defining the function has no side
effects as you still have to enable the mock. If the function was
already defined this method does nothing.