function Union(sequence a, sequence b)
    for i=1 to length(a) do
        if not find(a[i],b) then
            b = append(b,a[i])
        end if
    end for
    return b
end function

function Difference(sequence a, sequence b)
sequence res = {}
    for i=1 to length(a) do
        if not find(a[i],b)
        and not find(a[i],res) then
            res = append(res,a[i])
        end if
    end for
    return res
end function

function Symmetric_Difference(sequence a, sequence b)
    return Union(Difference(a, b), Difference(b, a))
end function

sequence a = {"John", "Serena", "Bob", "Mary", "Serena"},
         b = {"Jim", "Mary", "John", "Jim", "Bob"}
?Symmetric_Difference(a,a)
?Symmetric_Difference(a,b)
?Symmetric_Difference(b,a)
?Symmetric_Difference(b,b)