\version "2.21.0"
\language deutsch

#(set! paper-alist (cons '("snippet" . (cons (* 100 mm) (* 70 mm))) paper-alist))
\paper {
  #(set-paper-size "snippet")
  tagline = ##f
  indent = 30
}

% I have files with defined orchestral voices, like below:

trumpet_first = \relative c'' {
	c d e f
}

trumpet_second = \relative g' {
	g a h c
}

%{
I have a score template for a whole orchestra. To keep this example
small I will only show trumpet's staff. The problem I have is that
eventually I want to extract voices from my score -- I can do this with
tags -- but the voices still are engraved with stems pointed up or
down (it depends if it is 1st or 2nd voice). I don't know how
to fix it and I would ask you for suggestions and ideas to undo the \voiceOne
or \voiceTwo commands.

I really like how tags are working, so I would like to stick with them.
Or maybe there is a scheme function to call a voice, for example trumpet_second,
by a symbol?:

\score {
 #(call-voice-by-symbol 'trumpet_second)
}
%}

orchestral_score = <<
	\new Staff = "Trumpets" <<
		\set Staff.instrumentName = "Trumpet I II in B"
		\set Staff.shortInstrumentName = "Tr"
		\set Staff.midiInstrument = #"trumpet"
		\new Voice = "Trumpet I" { \voiceOne \transposition b \tag #'tag_trumpet_first \trumpet_first }
		\new Voice = "Trumpet II" { \voiceTwo \transposition b \tag #'tag_trumpet_second \trumpet_second }
	>>
>>

% This is how my orchestral score is engraved and it is fine:
\score {
	\orchestral_score
}

% This is a voice extracted with a tag and the result is not satisfying me.
\score {
	\keepWithTag #'tag_trumpet_second \orchestral_score
		\layout {
		\context {
			\Staff
			\remove Instrument_name_engraver
		}
	}
}

% I want it to look like this.
\score {
	\trumpet_second
}