Require implementations to raise a compile time error when list
is used with an unsupported test.
That's already what happens when there are such errors in scripts.
Why should we say it explicitly here, and not everywhere else?
Implementations MAY support other tests but MUST raise a compile
time error when a script contains any standard test where support has
not been explicitly specified.
Not explicitly specified by whom? How?
How does this make scripts portable? This seems to encourage private
extensions, which has really become a thorn in the web.
Barry