Before revision:  Finally, we consider a structure-approximating relaxation of the design, and provide a \Theta(n) algorithm which, given a structure S that avoids two trivially non-designable motifs, transforms S into a designable structure constructively by adding at most one base-pair to each of its stems . Acknowledgements The authors would like to thank C\'edric Chauve (Simon Fraser University) for fruitful discussions and constructive criticisms. YP is greatly indebted to the French Centre National de la Recherche Scientifique and the Pacific Institute for the Mathematical Sciences for funding an extended visit at the Simon Fraser University .
After revision:  Finally, we consider a structure-approximating relaxation of the design, and provide a \Theta(n) algorithm which, given a structure S that avoids two trivially non-designable motifs, transforms S into a designable structure constructively by adding at most one base-pair to each of its stems .