I'm trying to understand what is the meaning of this statement.
A088518 Symmetric secondary structures of RNA molecules with n nucleotides. 4
%N Symmetric secondary structures of RNA molecules with n nucleotides.
%C Diagonal sums of triangle in A088855. [From Philippe Deléham, Jan 04 2009]
%C Number of prime symmetric Dyck (n+2)-paths with no ascent of length 1. E.g. the a(3) = 2 5-paths are
UUUUUDDDDD and UUUDDUUDDD. - David Scambler, Aug 27 2012