A sequence is a type of set where the elements have been assigned offsets (a integral ordering). If the same element is assigned multiple offsets, then the sequence "revisits" the element for each assigned offset.  Logically, we can think of a sequence as a set of pairs <element :T, offset : Integer>. Since both element and offset must be equal for the ordered pair to be equal, there is no violation of the set semantics.  Not all numbered slots in a sequence must be occupied, and no offset may be occupied that exceeds the "length." <br /></p><p><br /></p><p>{offset > length implies element.isNull}<br /></p>