add lessons file

lessons.md

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+Things I would do differently with hindsight:
+
+* Make pointed dependent maps primitive:
+```lean
+structure ppi (A : Type*) (P : A → Type) (p : P pt) :=
+  (to_fun : Π a : A, P a)
+  (resp_pt : to_fun (Point A) = p)
+```
+(maybe the last argument should be `[p : pointed (P pt)]`). Define pointed (non-dependent) maps as a special case.
+Note: assuming `P : A → Type*` is not general enough.
+
+* Use squares, also for maps, pointed maps, ... heavily
+
+* Type classes for equivalences don't really work
+
+* Coercions should all be defined *immediately* after defining a structure, *before* declaring any
+  instances. This is because the coercion graph is updated after each declared coercion.
+
+* [maybe] make bundled structures primitive