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The objective of this paper is to study, by new formal methods, the notion of tree code introduced by Nivat in (Tree Automata and Languages, Elsevier, Amsterdam, 1992, pp. 1-19). In particular, we consider the notion of stability for sets of trees closed under concatenation. This allows us to give a characterization of tree codes which is very close to the algebraic characterization of word codes in terms of free monoids. We further deÿne the stable hull of a set of trees and derive a defectdoi:10.1016/s0304-3975(99)00317-5 fatcat:7og4auczyvbmjlhu35aimkkgbu