3 The reasoning is based on the observations that in the arc-eager system, new arcs (h, l, m) can be derived iff the following conditions hold: (a) There is no existing arc (h , l , m) such that h = h, and (b) Either both h and m are on the buffer, or one of them is on the buffer and the other is on the stack.
: (a) once a word acquires a head (in a Left-Arc or Right-Arc transition) it loses the ability to acquire any other head.