# let a = [ "John"; "Bob"; "Mary"; "Serena" ]
  and b = [ "Jim"; "Mary"; "John"; "Bob" ]
  ;;
val a : string list = ["John"; "Bob"; "Mary"; "Serena"]
val b : string list = ["Jim"; "Mary"; "John"; "Bob"]

# a -|- b ;;
- : string list = ["Jim"; "Serena"]

# a -| b ;;
- : string list = ["Serena"]

# b -| a ;;
- : string list = ["Jim"]