data InterfaceOnPositions : AddCont -> (Type -> Type) -> TypeConvenience datatype storing the property that
an additive container `c` has an interface `i` on its positions
MkI : ((s : c .Shp) -> i (c .Pos s)) => InterfaceOnPositions c iFor every shape s the set of positions c.Pos s has that interface
data IsFlat : AddCont -> Type