Concatenating lists using <>

If two lists are to be joined together, the general-purpose Pop-11 concatenator can be used, as in

    vars list1 = [a b c], list2 = [d e f];
    list1 <> list2 =>
    ** [a b c d e f]

    list1 <> list2 <> list1 <> list2 =>
    ** [a b c d e f a b c d e f]