  A combinatorial Gray code for a set of combinatorial objects is a sequence of
all combinatorial objects in the set so that each object is derived from the
preceding object by changing a small part.
  In this paper we design a Gray code for ordered trees with n vertices such
that each ordered tree is derived from the preceding ordered tree by removing a
leaf then appending a leaf elsewhere. Thus the change is just remove-and-append
a leaf, which is the minimum.
