-- The “edit graph” for sequences a and b corresponds to items:
-- in a on the horizontal axis
-- in b on the vertical axis
--
-- Given a-coordinate of a point in a diagonal, you can compute b-coordinate.
--
-- Forward diagonals kF:
-- zero diagonal intersects top left corner
-- positive diagonals intersect top edge
-- negative diagonals insersect left edge
--
-- Reverse diagonals kR:
-- zero diagonal intersects bottom right corner
-- positive diagonals intersect right edge
-- negative diagonals intersect bottom edge