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atom type question

    Date: Tuesday, 12 May 1987  06:00-EDT
    From: Christopher Fry <cfry at OZ.AI.MIT.EDU>
    Re:   atom type question

    Is ATOM supposed to be true of non-COMMON data types as well?

I think the language in the manual pretty clearly indicates that the
answer is yes, although there currently isn't much discussion of
extensions to the type system.  CONS is a subtype of COMMON and ATOM =
(NOT CONS), therefore anything not COMMON must be an ATOM.  

In can't see any reason for wanting thing to be otherwise.  If you
want to add some new object that acts like a cons but isn't, then you
can make it a subtype of cons.  Although CLTL says that
"implementations may not unilaterally add subtypes to COMMON", I think
that there is no way for a correct program to tell that you have added
a subtype except in the cases where CLTL says that some types form an
exhaustive partition of another type.