[sciurius]

Good work, @erhe, thanks for providing these excellent reference materials.

I think that in case of MobileSheets and ChordPro, the much simpler yet effective approach that Mike mentioned in #14 will suffice.

Note that there is a difference between key transpose, and interval (semitones) transpose. ChordPro uses semitones transposition. This means that it is not possible to transpose from e.g. F# to Gb since the interval is zero. You can, however transpose from F# to G and then from G to Gb. But that is different matter.