Sequence is an enumerated collection of objects in which repetitions are allowed and order matters.
Like a set, it contains members (also called elements, or terms).
The number of elements (possibly infinite) is called the length of the sequence.
Unlike a set, the same elements can appear multiple times at different positions in a sequence,
and unlike a set, the order does matter.
Formally, a sequence can be defined as a function whose domain is either the set of the natural numbers
(for infinite sequences), or the set of the first n natural numbers (for a sequence of finite length n).
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