Andrzej Ehrenfeucht Tero Harju Paulien ten Pas Grzegorz Rozenberg Permutations, parenthesis words, and Schröder numbers. 259-264 1998 190 Discrete Mathematics 1-3 http://dx.doi.org/10.1016/S0012-365X(98)00155-1 db/journals/dm/dm190.html#EhrenfeuchtHPR98