"is an error" -> "is undefined"

[Guillermo J. Rozas <jinx@hpesogg.hp.com>]

    I was wrong to be so restrictive.  Kent M Pitman is right about the
    alternative possibility of you proposing new notation and terminology
    to cover this case.  That is, maybe by replacing the phrase "is an
    error" with "is undefined", and adding the appropriate definition of
    "is undefined", we will have a proposal acceptable to all.

Undefined is fine with me.  I previously suggested the possibility of
phrasing it as an implementation restriction.  That would also be
fine.

I definitely do not want anything about an error said.