[A comment rather than a question]: All  '-erir' verbs [apart from the barely used 'enjerir'] change the e to ie:, e.g. sugerir, referir, preferir, diferir, transferir, requerir and a host of others. A similar rule applies to [all !] '-vertir' verbs: convertir, divertir(se), etc. etc.... On the other hand, all '-etir', '-edir' and '-egir' verbs adopt the e>i change.... With help from a CD which enabled words to be listed in reverse-alphabetical order, I examined the contents of my CLAVE dictionary, and posted the results in http://dlmcn.com/ir_verbs.html (the accents are properly represented in http://DLMcN.com/irverbs.doc ).