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.


#macro( foo $a )
  foo :  \$b = $b
  #set ($b = $a * 2)
  foo after : \$b = $b
#end

#macro( bar $b)
  #woobie($b)
  bar post woobie : $b
#end

#macro( woobie $b)
  #foo($b)
   woobie post foo : $b
#end

#set($b = 0 )
#set($arg = 4)
precall \$b = $b
#bar( $arg)
postcall \$b =  $b