25 | 25 | Hierarchy-3 is also mathematically neat, since a topological (resp. differentiable) manifold is obviously a open subset of a topological (resp. differentiable) manifold. In this respect it reverses the logic of Hierarchy-1, where the class `OpenTopSubmanifold` inherits from `TopologicalManifold`, not the opposite. Maybe the latter logic is quite well spread for ''algebraic'' structures in Sage, I mean classes for substructures inheriting from classes for the ambient structure. But for ''topology'', the reverse logic, as proposed in Hierarchy-3, could be more adapted: a topological space is often treated as an open subset of itself. For instance, this occurs in its very definition: a topological space is a set X endowed with a collection of subsets of X, called the open subsets, such that the empty set and X are open, etc. |