Transitivity of coercions

[Guy Steele <gls@think.com>]

Note that an explicit decision was made early in the design of CL
not to make all coercions transitive.  For example, symbols
coerce to strings (for string functions), and strings are sequences
(and so can be mixed with other sequence types), but symbols are
not sequences.

If we cannot have consistency, let us then have consistency of
inconsistency.  (Also known as, "This screw-up was good enough
for grampa, and it's good enough for me!")

--Guy