  We introduce L-presentations: group presentations given by a generating set,
a set of relations and a set of substitution rules on the generating set
producing more relations. We first study in full generality the structure of
finitely L-presented groups, i.e. groups for which all the above sets are
finite, and then show that a broad class of groups acting on rooted trees admit
explicitly constructible finite L-presentations: they are the "branch" groups
defined by R. Grigorchuk.
