I have changed the problem text spotted by Will to fit with R5RS. The new
text in SRFI-13 is:

    If a procedure is said to return "unspecified," this means that nothing at
    all is said about what the procedure returns. Such a procedure is not even
    required to be consistent from call to call. It is simply required to
    return a value (or values) that may be passed to a command continuation,
    e.g. as the value of an expression appearing as a non-terminal subform of
    a BEGIN expression. Note that in R5RS, this restricts such a procedure to
    returning a single value; non-R5RS systems may not even provide this
    restriction.

There is almost identical text describing SRFI-14's CHAR-SET-FOR-EACH.
    -Olin