0 Martin, L.C. 2008 Folding Back and the Growth of Mathematical Understanding: Extending the Pirie-Kieren Theory Journal of Mathematical Behavior 27 1 64-85 http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6W5B-4SN8V59-1&_user=486651&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000023538&_version=1&_urlVersion=0&_userid=486651&md5=42fd8390dcb205bfc60e49ea73ad377a