PartialEquivBEq α says that the BEq implementation is a
partial equivalence relation, that is:
- it is symmetric: 
a == b → b == a - it is transitive: 
a == b → b == c → a == c. 
PartialEquivBEq α says that the BEq implementation is a
partial equivalence relation, that is:
a == b → b == aa == b → b == c → a == c.