fonttools/Tests/feaLib/data/ChainPosSubtable.fea

feature test {
    pos X [A - B]' -40 B' -40 A' -40 Y;
    subtable;
    pos X A' -111 Y;
    subtable;
    pos X B' -40 A' -111 [A - C]' -40 Y;
} test;