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Joshua Bloch, "Effective Java (2nd edition)" Item 28: page 137 :
"Do not use wildcard types as return
types. Rather than providing additional flexibility for your users, it would force
them to use wildcard types in client code.
Properly used, wildcard types are nearly invisible to users of a class. They
cause methods to accept the parameters they should accept and reject those they
should reject. If the user of a class has to think about wildcard types, there is
probably something wrong with the class’s API."
Fixes #75. ForbidWildcardAsReturnTypeCheck was introduced.