A multiple substitution replaces one glyph by a number of new glyphs. Use this, for example, to decompose ligatures.
lookup decompose {
sub ffi -> f f i;
sub fi -> f i;
}
At the left side of ->
you should supply one glyph, which is
not a list.
At the right side a sequence of glyphs should appear.
The right-hand side sequences may have variable length, as long as they are
at least two glyphs long.