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R1277 / 1:1

External Service specifies required input with exactly one External Signature

External Signature defines input into exactly one External Service]

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There's nothing special about an External Signature, it's just a set of zero or more parameter type pairs.

But an External Signature is custom fit to a single External Service, so if two External Services send the same set parameter type pairs, we'll still need to create a distinct signature for each. This is incontrast to State Signatures which are shared among States and Events to ensure the constraint that all Events entering a Real State share a common State Signature.