Possible marginally-incompatible change to array subscripting

I'm reviewing Yury Zhuravlev's patch to allow array slice boundaries to be
omitted, for example "a[4:]" means "the slice extending from element 4 to
the last element of a".  It strikes me that there's an improvement we
could easily make for the case where a mixture of slice and non-slice
syntax appears, that is something like "a[3:4][5]".  Now, this has always
meant a slice, and the way we've traditionally managed that is to treat
simple subscripts as being the range upper bound with a lower bound of 1;
that is, what this example means is exactly "a[3:4][1:5]".

ISTM that if we'd had Yury's code in there from the beginning, what we
would define this as meaning is "a[3:4][:5]", ie the implied range runs
from whatever the array lower bound is up to the specified subscript.

This would make no difference of course for the common case where the
array lower bound is 1, but it seems a lot less arbitrary when it isn't.
So I think we should strongly consider changing it to mean that, even
though it would be non-backwards-compatible in such cases.

Comments?
        regards, tom lane