For the record, S. Gueron has some papers on similar tree hashes (http://dx.doi.org/10.4236/jis.2014.53010 http://dx.doi.org/10.4236/jis.2014.53010) and there's

For the record, S. Gueron has some papers on similar tree hashes (http://dx.doi.org/10.4236/jis.2014.53010 http://dx.doi.org/10.4236/jis.2014.53010) and there's more theory in http://sponge.noekeon.org/TreeHashing.pdf http://sponge.noekeon.org/TreeHashing.pdf .