Extracting pitch of first note, then assign duration

[Matthew Fong]

Hello everyone,

I am in the process of writing a note replacement function that allows me
to place a custos anywhere I might need.

I am stuck on writing a function that extracts the pitch of the first note,
given a list of notes, and then forcing a duration of 1/4. Any pointers
would be appreciated.

For example given the list of notes below, I would like a function to
output c4.
{
  \clef treble
  c2 d e f
  g4 a b c
}


Many thanks,
mattfong