When the algorithm is in a configuration λ 1 |i, λ 2 , j|B, A , we will say that it is considering the focus words i and j, located at the end of the first list and at the beginning of the buffer.
if each arc in a tree-compatible set is individually reachable from configuration c, then that Shift: Right-Arc: only if k | k → j ∈ A (single-head) and j → * i ∈ A (acyclicity).