The "interaction with the external world" family contains which three patterns, and what shared problem do they address?

Oracle, Reverse Oracle, and Legal-and-Smart-Contract Pair — all bridge the gap between a sealed blockchain and the systems, data, and legal meaning that live outside it.

Because a smart contract can't see beyond the chain, an application that has to react to real-world facts or plug into existing software needs a deliberate bridge. The three patterns handle three directions of that bridge:

• Oracle — pulls outside state into the chain so contracts can act on it.
• Reverse Oracle — lets an existing off-chain system query the chain, integrating blockchain into legacy software non-intrusively.
• Legal-and-Smart-Contract Pair — binds an on-chain contract to an off-chain legal agreement so each is the authoritative counterpart of the other.

The recurring tension across all three is connectivity (link to the outside) balanced against trust or simplicity — every bridge you add re-introduces something you have to trust or change.