Newbie question: tie between a and b

[Gyula P. Szokoly]

Hi,

  GNU LilyPond 2.10.33, input is at the end. The tie between the two 'a's
works, but not between a and b. The problem seems to be related to the
orientation of the stem (i.e. tie between opposite stems) but forcing
the stem (\stemUp) does not seem to help. I am probably missing something
trivial...

  Any ideas? Same happens with Linux and Win (cygwin, w2000pro) versions.

Thanks,
  Gyula

\book {
  \paper {
  }

  \score {
    \relative c'' {
    \time 4/4
    a4 ~ a a ~ b }
  }
}



\version "2.10.0"  % necessary for upgrading to future LilyPond versions.