The global dictionary SD is available to store
   private data between repeated calls to the same function.
   The global dictionary GD is public data,
   that is available to all Python functions within a session;  use with
   care.
  
   Each function gets its own execution environment in the
   Python interpreter, so that global data and function arguments from
   myfunc are not available to
   myfunc2.  The exception is the data in the
   GD dictionary, as mentioned above.