VIII. Let p be an equivalence on a set X. A subset AC X is said to be a
transversal of p if each p-class contains exactly one element of A.
In the full transformation semigroup Tx, denote the H-class
$H_{A,p} = \{a \in T_X : im(a) = A, ker(a) = p\}$
Show that $H_{A,p}$ is a group if and only if A is a transversal of p.
Hint: you may use Green's theorem.