ERRATUM
Walks in the quarter plane: Kreweras' algebraic model
There is a shameful mistake on the second page, in the introduction.
The number of square lattice walks of length 2n confined in the
first quadrant, starting and ending at the origin is
b(2n) = 1/( (2n+1)(2n+4) ) binomial(2n+2,n+1) ^2.
That is, the term (2n+2) must be replaced by (2n+4).
This number is the product of the Catalan numbers C_n and C_{n+1}.
In [20], the number b(2n) is written as a sum of 4 terms, each of
them having a combinatorial explanation.
A (recursive) bijective explanation of the Catalan product appears in
Cori, Robert; Dulucq, Serge; Viennot, Gérard
Shuffle of parenthesis systems and Baxter permutations.
J. Combin. Theory Ser. A 43 (1986), no. 1, 1--22.
A more recent, non-recursive bijective proof was recently found by
Olivier Bernardi (http://www.labri.fr/bernardi/), whom I thank for
having spotted this mistake in my paper.