Bounds/Frees of (\x. (\y. x y z) (\z y. x y z) (\x. x y z))

Note the first x-binder scopes across the entire expression, so
x will not occur free.

Then for each (of the three) abstraction in the body, we can check
which of its occurences are free/bound.

1) The x and the y are bound.  The z is free.

2) All three variables are bound.

3) Only the x is bound.  The other two are free.  Note, however, that
it is the x in this abstraction (not the outer binder) which controls
this x.