The font for this test case has
an AAT Extended Glyph Metamorphosis Table
with a single Rearrangement Subtable. Its finite-state machine
contains one single state with the following transitions:
For glyph A, the machine sets both start
and end of the marked glyph sequence to the current
glyph position.
For glyph B, the machine changes the end of the
marked glyph sequence to the current position.
For any other glyph (“out of bounds”), the machine executes a
rearrangement action.
If your rendering system correctly implements Apple Advanced
Typography, it should swap the first two letters in ABXAB.