sequence <sub command> ?<arg> ...?
A sequence is a container which contains a sequence of objects. There are currently two types of sequences, a series and a repeat.
A series is an arithmetic progression.
A repeat is a repetition of one object the specified number of times.
% set a [sequence repeat 10 {}] {} {} {} {} {} {} {} {} {} {}
% set a [sequence series 1 10 4] 1 5 9 13 17 21 25 29 33 37
Both of these sequences have a very compact internal representation which is not expanded when used with the generic container commands.