No, I wanted: "If the count of <B>s for the interval is zero, the
interval is empty and I'm not interested;

what's the difference between "the count of B in the interval is zero"
and "the next element is a T not a B" it appears to be the same as far
as I can see, except the latter is easier to express in xpath.

Maybe a lack of precision, or too much verbosity, in my original post.
An interval is *not* defined in terms document position, but in terms of
the numbers in the text nodes attached to the <T> elements. I wrote:

    <Urmel>
      <T>0</T>
      <T>6</T>
      <T>12</T>
      <T>18</T>
      <T>24</T>
    [...]

    A <T> indicates a point in time. <T>s are ordered numerically.
    Direct siblings define an interval starting at the earlier <T> and
    ending before the later <T>.

Should have be more explicit here and state that the first interval goes
from 0 to 6, the second one from 6 to 12, and so on.