TrillSpanners not lining up

[David Stephen Grant]

Hello,

I'm having some trouble with TrillSpanners not lining up correctly. The manuals seem to indicate that two successive TrillSpanners should be able to line up vertically without any explicit \stopTrillSpan, but in the following example the second trill is printed higher than the first, as if avoiding a collision. Any ideas?

Best, David

http://www.lilypond.org/doc/v2.19/Documentation/notation/expressive-marks-as-lines#trills

\version "2.19.8"
{
  d''1\startTrillSpan |
  d''1\startTrillSpan |
  R1\stopTrillSpan |
}