CL-USER> (trace append-find)
(APPEND-FIND)

CL-USER> (append-find '? '(4 6) '(1 2 3 4 5 6))
  0: (APPEND-FIND ? (4 6) (1 2 3 4 5 6))
    1: (APPEND-FIND ? (4 6) (2 3 4 5 6))
      2: (APPEND-FIND ? (4 6) (3 4 5 6))
        3: (APPEND-FIND ? (4 6) (4 5 6))
          4: (APPEND-FIND ? (4 6) (5 6))
            5: (APPEND-FIND ? (4 6) (6))
              6: (APPEND-FIND ? (4 6) NIL)
              6: APPEND-FIND returned (THE LISTS INTRODUCED ARE INCOMPATIBLE)
            5: APPEND-FIND returned (6 THE LISTS INTRODUCED ARE INCOMPATIBLE)
          4: APPEND-FIND returned (5 6 THE LISTS INTRODUCED ARE INCOMPATIBLE)
        3: APPEND-FIND returned (4 5 6 THE LISTS INTRODUCED ARE INCOMPATIBLE)
      2: APPEND-FIND returned (3 4 5 6 THE LISTS INTRODUCED ARE INCOMPATIBLE)
    1: APPEND-FIND returned (2 3 4 5 6 THE LISTS INTRODUCED ARE INCOMPATIBLE)
  0: APPEND-FIND returned (1 2 3 4 5 6 THE LISTS INTRODUCED ARE INCOMPATIBLE)