Parallel music and \skip while working on a part

[Joel C. Salomon]

This may be obvious to some, but I've found this construction helpful
while working on parallel pieces of music, and I thought I'd share:

\version "2.23.80"

upper = \relative c' {
  c4 d e f |
}

lower = << \skip \upper \relative c {
  f e
  % still being worked on
}
        >>

\score {
  \new PianoStaff <<
    \new Staff = "upper" \upper
    \new Staff = "lower" \lower
  >>
}

And when the second part is complete, the `<< \skip \upper` and `>>`
can be removed.

—Joel