boost mpl if demonstration

Consider the following code adapted from Polukhin:

When run:

detail::pre_inc_functor::operator ()
x = 6
detail::plus_assignable_functor::operator ()
cls::operator +=
y = 90
detail::post_inc_functor::operator ()
post::operator ++
z = 141
post::operator ++
alpha = 141
z = 142