  Starting from a succession rule for Catalan numbers, we define a procedure
encoding and listing the objects enumerated by these numbers such that two
consecutive codes of the list differ only for one digit. Gray code we obtain
can be generalized to all the succession rules with the stability property:
each label $(k)$ has in its production two labels $c_1$ and $c_2$, always in
the same position, regardless of $k$. Because of this link, we define Gray
structures the sets of those combinatorial objects whose construction can be
encoded by a succession rule with the stability property. This property is a
characteristic that can be found among various succession rules, as the finite,
factorial or transcendental ones. We also indicate an algorithm which is a very
slight modification of the Walsh's one, working in a O(1) worst-case time per
word for generating Gray codes.
