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SXHASH on lists
At one time, SXHASH had a definition such that the hash of a
cons node was not determinable from the hash's of the car and cdr
of that node.  So we changed SAHASH, in order to make it possible
for the function to be written easily in LISP.  Admittedly, we should
have chosen to make it commutative in the car direction rather than
the cdr direction.  But changeing things like that is not so easy - 
the primary use of SXHASH is for list structure in FASL files, and
all those file have been comiled useing the existing algorithm,
incorporatig in the file the various sxhash numbers.  It wont
break the code to change the algorithm, but likely many many many
copies of a given constant expressions would be generated, whereas
now there is just one copy of a given s-expression  for all files