redim shared as string Result(-1)   'represent our sets as strings;
                                'this'll do to illustrate the concept

sub sym( A() as string, B() as string )
    dim as integer ai, bi, ri
    dim as boolean add_it
    for ai = lbound(A) to ubound(A)
        add_it = true
        for bi = lbound(B) to ubound(B)
            if A(ai) = B(bi) then
                add_it=false
                exit for    'if item is common to both lists, don't include it
            end if
        next bi
        if add_it then
            for ri = 0 to ubound(Result)
                if A(ai) = Result(ri) then
                    add_it=false
                    exit for
                    'if item is already in the result, don't include it again
                end if
            next ri
        end if
        if add_it then
            redim preserve as string Result(0 to ubound(Result)+1)
            Result(ubound(Result)) = A(ai)
        end if

    next ai
end sub

dim as string A(0 to 3) = {"John", "Bob", "Mary", "Serena"}
dim as string B(0 to 4) = {"Jim", "Mary", "John", "Bob", "Jim"}
                             'contains a double to show code can handle it


sym(A(), B())
sym(B(), A())

for i as uinteger = 0 to ubound(Result)
    print Result(i)
next i