We have also provided a functional interface to these relationships through the methods
addStaff and removeStaff in Branch and setBranch and getBranch in Staff. Note also the
transparency of the bidirectional relationships; for example, when we invoke the addStaff
method to specify that this branch (b1, say) Has the given member of staff (s1 say), the
inverse relationship WorksAt is also set up (that is, s1 WorksAt b1).