Returns the set of links between two sentences: A = {a1,a2,...an}
where ak = (k1, k2) is an integer pair representing the link
between word in position k1, in one sentence, and k2 the k2-th
word in the other sentence.
Returns the set of links between two sentences: A = {a1,a2,...an}
where ak = (k1, k2) is an integer pair representing the link
between word in position k1, in one sentence, and k2 the k2-th
word in the other sentence.