MProperty
subclass.the two specialization hierarchy are symmetric
redef type MPROPERTY: MAttribute
src/model/model.nit:2699,2--33
redef type MPROPERTY: MVirtualTypeProp
src/model/model.nit:2710,2--39
redef type MPROPERTY: MInnerClass
src/doc/model_ext.nit:106,2--34