||in the second part when talking about the graduation problem it says
It is important to understand that any order to iterate through set A can be considered when solving the standard bipartite-matching problem. For example, it doesn't matter what element from set A we choose to be the first one to be matched. Consider the solution found by the algorithm containing this element x from A, matched with an element y from B. Also, we should consider any optimal solution. Clearly, in the optimal, y must be matched with an element z from A, otherwise we can add the pair x-y to the matching, contradicting the fact that the solution is optimal. Then, we can just exchange z with x to come with a solution of the same cardinality, which completes the proof.
Let A have one element a. Let B have 2 elements b, c. Let there be the paths a->b and a->c. there are 2 maximal matchings a->b and a->c. pick one of them say a->b. x=a y=b. now in the other optimal solution a->c b is not matched but according to the proof it must be. am i missing something?