\version "2.19.83"

\paper { indent = 0 }
butNotDots = \revert Dots.transparent

musica = \relative c''{ f4 g << { \hideNotes a4.( \unHideNotes g8) } \\ { a4.( g8) } >> }
musicb = \relative c''{ f4 g << { a4.( g8) } \\ { a4.( g8) } >> }
musicc = \relative c''{ f4 g << { \stemDown a4.( \stemUp g8) } \\ { \hideNotes a4.( \unHideNotes g8) } >> }
musicd = \relative c''{ f4 g << { \hideNotes \butNotDots a4.( \unHideNotes g8) } \\ { a4.( g8) } >> }

\markup "a) initial code; hiding voiceOne"
\score {
  \musica
}
\markup "b) no hiding voices"
\score {
  \musicb
}
\markup "c) hiding voiceTwo and adjusting stems of voiceOne"
\score {
  \musicc
}
\markup "d) hiding voiceTwo and keeping Dots"
\score {
  \musicd
}