  We study actions of finitely generated groups on $\bbR$-trees under some
stability hypotheses. We prove that either the group splits over some
controlled subgroup (fixing an arc in particular), or the action can be
obtained by gluing together actions of simple types: actions on simplicial
trees, actions on lines, and actions coming from measured foliations on
2-orbifolds. This extends results by Sela and Rips-Sela. However, their results
are misstated, and we give a counterexample to their statements.
  The proof relies on an extended version of Scott's Lemma of independent
interest. This statement claims that if a group $G$ is a direct limit of groups
having suitably compatible splittings, then $G$ splits.
