The first and last messages on the answering machine could be the first and second messages left by which one of the ...

I don't understand why the 3 entities that must be included are H,L, T and not P, since the rule with F-->P and T (P--T) is identical to P--->H and L (H--L). I deduced that those 4 entities must be included, and it is not explained why both H and L are carried over, but only T is on the other side.