This tests to make sure that we only alter either
the local arg in a VM, or the global context.

So in foo, we are accessing the global 'b', and 
the fact that we were called from bar that has it's
arg of 'b' is irrelevant.


precall $b = 0
      foo :  $b = 0
    foo after : $b = 8
   woobie post foo : 4
  bar post woobie : 4
postcall $b =  8