to diff :a :b [:acc []]
  if empty? :a [output sentence :acc :b]
  ifelse member? first :a :b ~
    [output (diff butfirst :a  remove first :a :b  :acc)] ~
    [output (diff butfirst :a  :b    lput first :a :acc)]
end

make "a [John Bob Mary Serena]
make "b [Jim Mary John Bob]

show diff :a :b   ; [Serena Jim]