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base64.cl contains an example of a bidirectional stream that converts data using the base64 part of MIME encoding. The current documentation

To use the example, start an Allegro Common Lisp and note the following file exists with the following contents:

cl-user(1): (shell "cat country.txt")
Now is the time
for all good people
to come to the aid
of their country.
0
cl-user(2): :cl base64
;;; Compiling file base64.cl
; Fast loading from bundle code/iodefs.fasl.
;   Fast loading from bundle code/iordefs.fasl.
;     Fast loading from bundle code/efmacs.fasl.
;;; Writing fasl file base64.fasl
;;; Fasl write complete
; Fast loading
;    .../base64.fasl
cl-user(3):

Now make a mime-encoded file. This example provides a lisp version of the base64 encoding. An older unix program called mpack used to be available, but it does not exist for some systems:

cl-user(3): (mpack "country.mime" "country.txt")
nil
cl-user(4): (shell "cat country.mime")
Content-Transfer-Encoding: base64

Tm93IGlzIHRoZSB0aW1lCmZvciBhbGwgZ29vZCBwZW9wbGUKdG8g
Y29tZSB0byB0aGUgYWlkCm9mIHRoZWlyIGNvdW50cnkuCg==
0
cl-user(5):

Now, open the mime-encoded file, and set up a base64 encapsulating stream on it:

cl-user(5): (setq yyy (open "country.mime"))
#<file-simple-stream #P"country.mime" for input pos 0 @ #x10000e63f72>
cl-user(6): (setq xxx (make-instance 'base64-reader-stream :base-stream yyy))
#<base64-reader-stream
   for input fd #<file-simple-stream #P"country.mime" for input pos 0>
  @ #x10000e67f02>
cl-user(7):

The stream is now ready for reads:

cl-user(7): (read-line xxx)
"Now is the time"
nil
cl-user(8): (read-line xxx)
"for all good people"
nil
cl-user(9): (read-line xxx)
"to come to the aid"
nil
cl-user(10): (read-line xxx)
"of their country."
nil
cl-user(11): (read-line xxx)
Error: eof encountered on stream
       #<base64-reader-stream
          for input fd #<file-simple-stream
                         #P"country.mime" for input pos 137>
         @ #x10000c7c6b2>
  [condition type: end-of-file]

Restart actions (select using :continue):
 0: Return to Top Level (an "abort" restart).
 1: Abort entirely from this (lisp) process.
[1] cl-user(12):

Note that file-position on the encapsulated stream will return nil (because it is a dual-channel stream, whose default device-file-position method always returns nil). It would be possible to extend this example to respond to file-position and also to set the file-position, but the file-position calculation would be very complex and the only useful value to set the file-position to would be 0, unless starting file-positions were recorded as each chunk is read in (or unless multiple reads were acceptable in determining the file-position to set).