Prototype and the specs go hand in hand. Write a spec - prove you can implement it. Write an implementation - write a spec so we can talk about what is important (vs details of how you implemented it but someone else is allowed to implement differently) Often parts that are "obvious" are not implemented, or only the trivial version is implemented. You need to do both.
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I would expect them to build prototypes to iterate over the specs.
Prototype and the specs go hand in hand. Write a spec - prove you can implement it. Write an implementation - write a spec so we can talk about what is important (vs details of how you implemented it but someone else is allowed to implement differently) Often parts that are "obvious" are not implemented, or only the trivial version is implemented. You need to do both.