To connect a particular user routine to be called by FZIN or FZOUT for this 'file':
EXTERNAL UserSR
with LUN: stream number
UserSR: name of the user routine
dummy: the third parameter is not at the moment used,
but it must be present in the call
This must be called after the 'file' has been initialized
with FZFILE, and before it is used with FZIN or FZOUT.
Different user routines may be connected by recalling this
routine any number of times;
see section 
for explanations.