Pulido Rull, Marco Antonio, Cindy Hernández, Lilian M. Peña, Nora M. Ponce, and Monserrat Rebolledo. 2010. “The Operant Conditioning of Letter String Problem Solving”. Mexican Journal of Behavior Analysis 36 (3):55-76. https://doi.org/10.5514/rmac.v36.i3.21375.