Lyric extenders and \skip

[Davide Liessi]

Hi.

Please consider the following example.

\version "2.19.15"
<<
  \relative { c' d e f }
  \addlyrics { a __ _ \skip 1 a }
  \relative { c' d e f }
  \addlyrics { a __ _ a a }
  \relative { c' d r f }
  \addlyrics { a __ _ \skip 1 }
  \relative { c' d r f }
  \addlyrics { a __ _ a }
>>

I would expect the length of the lyrics extenders lines in staves 1 and 
3 to be the same as in staves 2 and 4.
Instead in staves 1 and 3 the extender line covers also the note 
corresponding to `\skip 1`.

Is this behaviour documented?
I couldn't find it in the manuals.

How can I achieve the desired result, i.e. a melisma with lyric extender 
line followed by a note with no syllable and no extender line?

Best wishes.
Davide