feature SUB6 {
    lookup GSUB6f1 {
        sub one two three' four' five six seven by X;
        sub two one three' four' six five seven by Y;
    } GSUB6f1;

    # format 2 is generally less efficient than format 3 and hard to generate
    # (as in, I'm not aware of any input that will compile to format 2)
    lookup GSUB6f3 {
        sub [space comma semicolon] e' by e.2;
    } GSUB6f3;
} SUB6;

# can't declare in a feature or it gets added to the feature
lookup MY_RULES {
    sub f by f.2;
} MY_RULES;

feature SUB5 {
    lookup GSUB5f1 {
        sub a' b' by a_b;
        sub c' d' by c_d;
    } GSUB5f1;

    lookup GSUB5f3 {
       sub f' lookup MY_RULES g';
    } GSUB5f3;
} SUB5;