The RNA shape abstraction transforms a concrete Vienna-Dot-Bracket string into a more abstract shape string. Usually, squared brackets condense rows of nested base-pairs (stems). For pseudoknots, we have to employ several different types of brackets to discriminate between the crossing stems. For reasons of generallity, we define symbols used for indicating stems in this external file "shapes.hh" for the normal case and e.g. in "pknot_shape.hh" for pseudoknot programs.