sumo#unique_list  a list in which no item appears more than once, i.e. a list for which there are no distinct numbers ?n1 and ?n2 such that (sumo#list_order_fn ?list ?n1) and (sumo#list_order_fn ?list ?n2) return the same value
  supertype:  sumo#list  a particular ordered n-tuple of items; generally speaking, lists are created by means of the list_fn function, which takes any number of items as arguments and returns a list with the items in the same order; anything, including other lists, may be an item in a list; note too that lists are extensional - two lists that have the same items in the same order are identical; note too that a list (the null_list) may contain no items
  subtype:  kif#null__null_list  type of empty lists
     instance:  sumo#null_list
  subtype:  kif#single__list_with_1_element  list of length 1

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