user_eq_dummy.m:014: Error: the type `foo'/0 contains no information, and as
user_eq_dummy.m:014:   such it is not allowed to have user-defined equality or
user_eq_dummy.m:014:   comparison.
user_eq_dummy.m:014:   Discriminated union types whose body consists of a
user_eq_dummy.m:014:   single zero-arity constructor cannot have user-defined
user_eq_dummy.m:014:   equality or comparison.
user_eq_dummy.m:018: Error: the type `bar'/0 contains no information, and as
user_eq_dummy.m:018:   such it is not allowed to have user-defined equality or
user_eq_dummy.m:018:   comparison.