Provided herein are libraries of nucleic acid species each comprising a transcription unit having a promoter region operatively linked to a coding sequence. The coding sequence of each nucleic acid species encodes a RNA cleavage substrate comprising a unique compomer species and a cleavage site. Each compomer species has a molecular mass distinguishable from the molecular mass of other compomer species in the library, and cleavage at a cleavage site releases a polynucleotide comprising the compomer species from the RNA cleavage substrate.