----------------------------------------
# # # 
2 1 0 
....main....
[ext] H  A type
[ext] H  B type
[ext] H  C type
====
[ext] H; v: A  B  C; v1: A  A  B

BY thinT 2 thenT selT 1 (dT 0)
# 1. [ext] H; v1: A  B type
# 2. [ext] H; v1: A  A

++++++++++
- : unit = ()
# 