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Direct access input from character streams.

  I should like to be able to read from a character stream after specifying
a position at which the next read (stream-tyi) function is to commence.  The
reason I should like to do this is that I have a large data base that is
conveniently representable as a collection of s-expressions, but which is
too large to keep in main memory.  The index for this collection, however,
is sufficiently small that it can easily fit into main memory.  Thus, I
could access the index to discover the position in the file at which the
related s-expression is stored, position the file to that position, and
read the s-expression with the lisp reader.
  There seems to be nothing in standard common lisp, nor in Allegro common
lisp that allows specifying the position in a stream at which the next
stream-tyi function is to commence.  Is there a foreign function in some
well-known (or arcane but accessible) library that I may use to replace
the system stream-tyi with one that allows the file position to be

  Thank you for your attention.

Van Snyder