This is a GlyphsApp extension, (partially) documented [here][1], but it is a
useful one as it allows concise glyph insertion using decomposition
without having to maintain the list of glyphs in two places. E.g.
```fea
@upper = [A-Z];
lookup insert {
sub @upper by @upper connector;
} insert;
feature ccmp {
sub @upper' lookup insert x;
} ccmp;
```
As apposed to:
```fea
@upper = [A-Z];
lookup insert {
sub A by A connector;
sub B by B connector;
sub C by C connector;
sub D by D connector;
sub E by E connector;
sub F by F connector;
sub G by G connector;
sub H by H connector;
sub I by I connector;
sub J by J connector;
sub K by K connector;
sub L by L connector;
sub M by M connector;
sub N by N connector;
sub O by O connector;
sub P by P connector;
sub R by R connector;
sub S by S connector;
sub T by T connector;
sub U by U connector;
sub V by V connector;
sub W by W connector;
sub X by X connector;
sub Y by Y connector;
sub Z by Z connector;
} insert;
feature ccmp {
sub @upper' lookup insert x;
} ccmp;
```
1. http://handbook.glyphsapp.com/en/layout/multiple-substitution-with-classes/
