A multiple substitution specifies a number of glyphs that may replace one other glyph. This may be used to allow the user a choice of variant glyphs.
lookup decompose {
sub e -> [e e.alt1 e.alt2];
sub z -> [z z.alt1 z.alt2];
}
At the left side of ->
you should supply one glyph, which is
not a list.
At the right side one list of glyphs should appear that contains the source
glyph and at least one other glyph.