Multiple alternates for opentype features (swash)

[oneweioranother]

I have a typeface which has swashes for some glyphs, some of these swashes have an alternate. i.e.

A, A.swsh, A.swsh.001

These swashes are only for the beginning or end of words.

Doing research I’ve found pages on Typophile describing how to deal with this with contextual alternates and so on, such as this page which mentions:

Single substitution rules ("by") and alternate substitution rules ("from") can't be used in the same feature/lookup. So if you're using classes to minimize the code for single substitutions (as in the example from the website), you'll need to create separate lookups. For example: feature salt { # Stylistic Alternates lookup single { sub @salt1 by @salt2; } single; lookup alternate { sub a from [a.alt1 a.alt2 a.alt3 a.alt4]; } alternate; } salt;

How can I implement this in Glyphs app? I've never seen mention of writing 'lookup' as I assume Glyphs has simplified Opentype feature code?

[Edit: Just write it in the feature panel as such without the ‘feature’ lines]

[oneweioranother]

(I just found http://www.glyphsapp.com/tutorials/features-part-4-positional-alternates but it doesn’t touch on when the letters are part of another feature and do not need to be activated automatically, i.e. swashes)

Found some other Typophile resouce, will post results.

[oneweioranother]

Best links for such a feature are:

Mark Simonson’s comment

and Karsten Luecke.

However I tried to do one for my swash that has multiple alternates for it’s beginning form like such:

@call = All letters
@cswsh1 = letters in its normal form which have swash initial forms
@cswsh2 = swash initial forms of cswsh1
@cswsh3 = letters in its normal form which have swash final forms
@cswsh4 = swash initial forms of cswsh3

lookup cswshinit { ignore sub @call @cswsh1'; sub @cswsh1' by @cswsh2; } cswshinit;

lookup cswshm {
ignore sub @call M’;
sub M’ from [M.swsh M.swsh.001];
} cswshm;

lookup cswshfina {
ignore sub @cswsh3’ @call;
sub @cswsh3’ by @cswsh4;
} cswshfina;

and get this error

Error: "Contextual alternate rule not yet supported" in Feature cswh in line: 8

So this is not possible? Whats the best way to allow access to the alternate M then? Just include it in swsh?

[mekkablue (Rainer Erich Scheichelbauer)]

What exactly are you trying to achieve in the sub M from… Line?

[oneweioranother]

I realise that for contextual swash maybe it doesn’t make sense as the user isn’t choosing the swash character. I wanted to make sure that the alternates were listed for M if a user wanted to choose their own variant, I just added it to the swsh feature instead.