Due to monotonicity, there is a natural pairing between the Right-Arc and Reduce actions and the Shift and Left-Arc actions: a word which is pushed into the stack by Right-Arc must be popped using Reduce, and a word which is pushed by Shift action must be popped using Left-Arc.
of this pairing, a Right-Arc move determines that the head of the pushed token must be to its left, while a Shift moves determines a head to its right.