implied contracts in the mapping functions?

[David A. Moon <Moon%SCRC-TENEX@MIT-MC>]

    Date: 17 September 1983 21:12 EDT
    From: Glenn S. Burke <GSB @ MIT-ML>
    I would appreciate comments on the following "bug report" i just
    received.

    I am iterating over a list and simultaneously adding things to the end of
    the list with NCONC.  It works fine until I get down to the final iteration.

I don't think the language should define what happens in this case (even though
it "works" in my implementation, as it happens).  Such iterations should be
written in terms of lower-level primitives, such as DO or TAGBODY+SETQ.