; num_class <- tcLookupClass numClassName
; is_str_class <- tcLookupClass isStringClassName
; let deflt_clss | ovl_str = [num_class, is_str_class]
| otherwise = [num_class]
; num_class <- tcLookupClass numClassName
; is_str_class <- tcLookupClass isStringClassName
; let deflt_clss | ovl_str = [num_class, is_str_class]
| otherwise = [num_class]