Walter,
  Le grammatica de interlingua es basate super un principio differente que
le principio del prototypo basate super tres variantes que vos ha
describite.

  In le introduction al _Interlingua Grammar_, Gode e Blair diceva:

"...every grammatical feature which is encountered in all the source languages
shall be retained in the grammar of the interlingua, or negatively, no
grammatical feature shall be so retained if it is missing from as much as one
of the source languages".


<"...tote characteristica que es incontrate in tote le linguas a base es
retenite in le grammatica del interlingua, o negativemente, necun characteristica
grammatica essera retenite si illo es absente de al minus un del linguas a
base."

Stan Mulaik