IfA <: B andB <: A, thenA = B (subtyping is antisymmetric)
A =:= A for allA (equality is reflexive).
A =:= A for allA (equality is reflexive). This also provides implicit viewsA <:< B whenA <: B, because(A =:= A) <: (A <:< A) <: (A <:< B).