Footnote spanner is not fully contained in parent spanner error

[Trevor Daniels]

I think I'm running into the same kind of problems David experienced in
tackling issue 2547 - too many of the examples I devise to explain how 
to use footnotes simply don't work as they should or at least as I expect.  

Here's another example:

\book {
  \header { tagline = ##f }
  \relative c'' {
    a4-\footnote #'(0 . 2) "Slur" (
    b8-\footnote #'(1 . -2) "Beam" [
    c-\footnote #'(1 . 1) "Tenuto" -- ]
    d2)
  }
}

This gives "Footnote spanner is not fully contained in parent spanner" 
and 'bounds of this piece aren't breakable' programming errors.
Move the terminating slur one note earlier or add a further note at the
end and it works fine.

Why?

Trevor