Common Lisp the Language, 2nd Edition
On the other hand, if it is specified in this book that in some situation ``an error is signaled,'' this means that:
In places where it is stated that so-and-so ``must'' or ``must not'' or ``may not'' be the case, then it ``is an error'' if the stated requirement is not met. For example, if an argument ``must be a symbol,'' then it ``is an error'' if the argument is not a symbol. In all cases where an error is to be signaled, the word ``signaled'' is always used explicitly in this book.
X3J13 has adopted a more elaborate terminology for errors,
and has made some effort to specify the type of error to be signaled
in situations where signaling is appropriate. This effort
was not complete as of September 1989, and I have made little
attempt to incorporate the new error terminology or
error type specifications in this book. However, the new terminology
is described and used in the specification of the
Common Lisp Object System appearing in chapter 28; this gives
the flavor of how erroneous situations will be described,
and appropriate actions prescribed, in the forthcoming ANSI Common
Lisp standard.