Full Idea
A 'sequence' of objects is like a set of objects, except that, unlike a set, the order of the objects is important when dealing with sequences. ...An infinite sequence satisfies 'x2 is purple' if and only if the second member of the sequence is purple.
Gist of Idea
A 'sequence' of objects is an order set of them
Source
Richard L. Kirkham (Theories of Truth: a Critical Introduction [1992], 5.4)
Book Reference
Kirkham,Richard L.: 'Theories of Truth: a Critical Introduction' [MIT 1995], p.154
A Reaction
This explains why Tarski needed set theory in his metalanguage.