  This paper, dating from May 1991, contains preliminary (and unpublishable)
notes on investigations about iteration trees. They will be of interest only to
the specialist.
  In the first two sections I define notions of support and embeddings for tree
iterations, proving for example that every tree iteration is a direct limit of
finite tree iterations. This is a generalization to models with extenders of
basic ideas of iterated ultrapowers using only ultrapowers.
  In the final section (which is most of the paper) I sketch a proof that any
tree iteration can be embedded into a normal iteration, that is, a tree
iteration with the extenders in nondecreasing order of strength and with
strictly increasing critical points.
