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We collect here common pitfalls and their solutions, patterns and tricks.

• Non Forgetful Inheritance is a nasty problem which can arise if a hierarchy is badly designed.
• Feather Factories are like factories but a bit quicker to declare.
• Missing Join is an error about an invariant HB wants you to keep. This page explains how to make HB happy.
• Abstraction Barriers are a useful tool to tame performance problems and define a clear API for the client of a library.
• Error Message FAQ. Ideally, error messages should be self explanatory... sometimes they are not.