syntax match bug: 'keepend' not officially supported, but required in some cases

The syntax documentation suggests that the 'keepend' argument applies
only to syntax regions, not syntax matches. But if I highlight a
buffer containing only the following line...

ABCDE

...with the following syntax definitions...

syn match A /A/
syn match Inside /[A-Z]\+/ transparent contained containedin=A
hi A guifg=blue

...the match group "A" extends (apparently incorrectly) from A through
E, though its pattern matches only the "A". Stranger still, one
additional nested "Inside" group is recognized in each successive
letter. I.e., "Inside" is contained within "Inside", in spite of the
`containedin=A'. Thus, running synstack on successive characters, I
see...

-- cursor on A --
A
Inside
-- cursor on B --
A
Inside
Inside
-- cursor on C --
A
Inside
Inside
Inside


I can fix the problem by adding the 'keepend' argument to the
definition for match group "A", but the "keepend" is highlighted as an
error by the vim syntax, and the help on :syn-match gives me no reason
to believe the argument should be supported.

Here are the results of synstack() with the following, "fixed" definition for A:
syn match A /A/ keepend

-- cursor on A --
A
Inside
-- cursor on B --
-- cursor on C --

Is 'keepend' meant to be supported for syn-match? The purpose of
'keepend' is to prevent a nested group from obscuring a match with a
containing group's "end pattern". Although a match group has no
explicit end pattern, the principle of checking for the end of the
match pattern certainly still applies, so I suppose it makes sense
that 'keepend' would be accepted for both syn-match and syn-region. If
that is the intent, then the documentation needs fixing.

As for the infinitely nested "Inside" groups, I can't account for
that, and suspect it's a bug, but perhaps I'm misunderstanding
something about contained and containedin?

Thanks,
Brett Stahlman

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