Inconsistent history expansion of characters adjacent to histchar

[Hauke Petersen <hkptrsn@gmail.com>]

zshexpn(1), heavily elided:

    HISTORY EXPANSION

    Following [the exclamation mark] is an optional event designator and then
    an optional word designator; _if neither of these designators is present,
    no history expansion occurs._

AFAICT, that's only partially true.

    % print !
    !
    % print !; print foo
    print print !; print foo
    print !
    foo

It does not immediately make sense to me why a single `!' would be
interpreted as a double `!!' before a `;' and tried, unsuccessfully,
to find an explanation.

    Event Designators
    !   Start a history expansion, except when followed by a blank, new-line,
    `=' or `('.  If followed immediately by a word designator, this forms a
    history reference with no event designator.

    [!!, !n, !-n, !str, !?str[?], !#, !{...}]

There is also this:

    a history reference with no event designator refers to the same event as
    any preceding history reference on that command line; if it is the only
    history reference in a command, it refers to the previous command

but that only applies to references with a word designator, which
there isn't in `!;'.  Nor does it satisfy the definition of any of the
event designators except `!str', but then it would a) eat the `;' as
being part of the reference, and b) fail with `event not found: ;' if
there is no previous command starting with ';'.  Like otherwise
similar `&':

    % print !& print foo
    zsh: event not found: &

Instead, as above, it expands like `!!;' which every once a while
makes my heart skip two beats when I realize what could have happened
in lines like the following with a previous command other than `:'.

    % :
    % ./foo & touch foo.$!; wait $!; rm foo.$!; printf \\a
    ./foo & touch foo.$:; wait $:; rm foo.$:; printf \\a
    [1] 3828
    wait: job not found: $:

I know expansion is always potentially destructive and, yes,
HIST_VERIFY could prevent the worst of it, but I do not think the
current behavior makes sense in the first place.  I'd expect and
prefer `!;'  not to be interpreted as a history reference, but seeing
the behavior of `!&', I think it should at least behave consistently.

(Sorry about the verbosity.)