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{{about|mathematical trees described by prefixes of finite sequences|trees described by partially ordered sets|Tree (set theory)}}
In [[descriptive set theory]], a '''tree''' on a set <math>X</math> is a collection of [[finite sequence]]s of elements of <math>X</math> such that every [[Prefix (computer science)|prefix]] of a sequence in the collection also belongs to the collection.
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A '''branch''' through a tree <math>T</math> is an infinite sequence of elements of <math>X</math>, each of whose finite prefixes belongs to <math>T</math>. The set of all branches through <math>T</math> is denoted <math>[T]</math> and called the '''''body''''' of the tree <math>T</math>.
A tree that has no branches is called '''''[[wellfounded]]'''''; a tree with at least one branch is '''''illfounded'''''. By [[
===Terminal nodes===
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