So, assume the operator f takes a sequence, and returns its elements as a set (without repetitions). For example if \mathbf{a} = (1,1,2,1,3,2) then f(\mathbf{a}) = \{1,2,3\} .
Now suppose we have a pair of sequences \mathbf {a} = (a_1,a_2,\cdots,a_n) and \mathbf {b} =...