  Any model of ZFC + GCH has a generic extension (made with a poset of size
aleph_2) in which the following hold:
  MA + 2^{aleph_0}= aleph_2+ there exists a Delta^2_1-well ordering of the
reals.
  The proof consists in iterating posets designed to change at will the
guessing properties of ladder systems on omega_1. Therefore, the study of such
ladders is a main concern of this article.
