fix #351 - arithmetic is slightly different for type arguments
fix #351 - arithmetic is slightly different for type arguments

jvasileff/ceylon.language,jvasileff/ceylon.language,unratito/ceylon.language,lucaswerkmeister/ceylon.language,lucaswerkmeister/ceylon.language,ceylon/ceylon.language,unratito/ceylon.language,ceylon/ceylon.language