Encode combining octave indicators for clefs as used by Sorabji and Finnissy

[dspreadbury]

Requested by Wilhelm von Hindenburger:

I am requesting an alternative version of the ottava clefs that is used by some composers, beginning with Kaikhosru Shapurji Sorabji. Instead of an 8 above a treble clef, for example, it is followed by Î (the Roman numeral 1 with an upwards-pointing caret, to indicate up one octave). For two octaves, it's ÎI, and for down an octave the caret is at the bottom and pointing downwards. (The same can be done for two octaves, but I don't know if Sorabji ever used that)

Here are examples:

https://www.youtube.com/watch?v=_OrAewTxBrc (Sorabji, Opus Clavicembalisticum)
https://www.youtube.com/watch?v=O8VSRTl9tfw (Michael Finnissy, English Country Tunes)

Note that the way that the Î transpositions can be added in the middle of the line might not be the easiest to add, and I think just making new alternative 8va symbols of the clef attached to the Î is fine.

[dspreadbury]

@mscuthbert says:

I remember seeing these. The pieces are decently well known. In one sense they could be thought of as stylistic alternates of 8va/8vb/15ma/15mb etc. but their placements are very different (and don’t allow an extension spanner or repetition each line). I think of them as clef8va alternates but again they can be used without a clef and they are invariant for whatever clef they are attached to. Maybe they are their own thing? I can’t think of a single SMuFL musical symbol that they have an exact semantic equivalent to. They might be stylistic variants of clefs and also stylistic variants of Ottava signs also. 

Usage is probably about equal to some of the middling common microtonal accidentals already accepted.

[dspreadbury]

It feels to me as if these symbols should not be indicated as stylistic alternates for octave clefs, since you could in theory use these markings with any clef (even if these two specific works only happen to use them with treble G and bass F clefs). So I think it would make sense to encode them somehow independently, indicating that they could be combined with clefs or indeed used on their own if the overall clef type is remaining the same and only the octave transposition is changing.

[ghost]

"An unusual procedure is the systematic replacement of the traditional octava sign (8va) by the letter I with a caret above (Î) to indicate that everything that follows (up to the indication loco, or a downward-pointing arrow in the early works) should be played an octave higher. Sorabji’s practice avoids the need for dotted lines on page after page. Indeed, in works written on three staves and more, the Î symbol is present throughout. Some composers—Ronald Stevenson, Michael Finnissy, Alistair Hinton, and Chris Dench among them—have followed Sorabji’s example."
Marc-André Roberge, Opus Sorabjianum, page 15 (abridged)

[dspreadbury]

These glyphs will be added to the Clefs supplement range as follows:

clefCombining8vaAltaSorabji	U+ED71
clefCombining15maAltaSorabji	U+ED72
clefCombining8vaBassaSorabji	U+ED73
clefCombining15maBassaSorabji	U+ED74