feat: introduce "cache" for derived model information

song.go

@@ -2,7 +2,6 @@ package sointu
 
 import (
 	"errors"
-	"iter"
 )
 
 type (
@@ -336,32 +335,3 @@ func TotalVoices[T any, S ~[]T, P NumVoicerPointer[T]](slice S) (ret int) {
 	}
 	return
 }
-
-func (s *Song) InstrumentForTrack(trackIndex int) (int, bool) {
-	voiceIndex := s.Score.FirstVoiceForTrack(trackIndex)
-	instrument, err := s.Patch.InstrumentForVoice(voiceIndex)
-	return instrument, err == nil
-}
-
-func (s *Song) AllTracksWithSameInstrument(trackIndex int) iter.Seq[int] {
-	return func(yield func(int) bool) {
-
-		currentInstrument, currentExists := s.InstrumentForTrack(trackIndex)
-		if !currentExists {
-			return
-		}
-
-		for i := 0; i < len(s.Score.Tracks); i++ {
-			instrument, exists := s.InstrumentForTrack(i)
-			if !exists {
-				return
-			}
-			if instrument != currentInstrument {
-				continue
-			}
-			if !yield(i) {
-				return
-			}
-		}
-	}
-}