  Since 6 has no children, d(6) must be ∅.  Similarly, d(5) and d(2) are also ∅.  d(4) = {d(6)} = {∅}.  d(3) = {d(4), d(5)} = { {∅}, ∅}.  And d(1) = {d(2), d(3)} = {∅, { {∅}, ∅} }. Note this is the set x with which we started.  This is no accident, and you are encouraged to think about why this is true.  A related point: for a graph like the one in equation (8), where we use the sets involved as the nodes of the graph, you should check that the identity function is a decoration.