Rules are principles, not principals

See these articles in TermiumPlus:

* http://www.btb.termiumplus.gc.ca/tcdnstyl-srch?lang=eng&srchtxt=principal&cur=8&nmbr=8&lettr=12&info0=12.03
* http://www.btb.termiumplus.gc.ca/tcdnstyl-srch?lang=eng&srchtxt=principal&cur=4&nmbr=8&lettr=3&info0=3.05