Before revision:  To match the experimental findings of the statistical behavior, we regard the substructure as a grand canonical ensemble and find its fugacity value corresponding to the number of stems . We also suggest a hierarchical picture based on the planar structure so that the non-planar structure such as pseudoknot are included .
After revision:  To match the experimental findings of the statistical behavior, we regard the substructure as a grand canonical ensemble and find its fugacity value corresponding to the number of stems .