| > Basically I need to represent a mapping from the set of the n first
| > positive integers to itself, with n ~= (expt 10 6).  I have try with
| > an hash-table and with an array, but I run out of memory.
| 
| An array of the integers between 0-1,000,000 shouldn't be hard to
| handle.  You should tell MAKE-ARRAY that you want to store small
| integers (called 'fixnums', since they use only a fixed number of
| bits).  So you would say:
| 
| (make-array (expt 10 6) :element-type 'fixnum)

if this is under ACL for Windows, remember to check the values of
MOST-POSITIVE-FIXNUM and ARRAY-DIMENSION-LIMIT.  IIRC, they are _tiny_.

#\Erik