(The optional seeds passed to each reset may differ.)
The chain of applications from reset to get_hash_value must be done in a single-threaded manner (you can't use fold_* on a state that's been used before)
More precisely, alloc () creates a new family of states. All functions that take t and produce t return a new state from the same family.
At any point in time, at most one state in the family is "valid". The other states are "invalid".
The state returned by alloc is invalid. The state returned by reset is valid (all of the other states become invalid). The fold_* family of functions requres a valid state and produces a valid state (thereby making the input state invalid). get_hash_value requires a valid state and makes it invalid.
These requirements are currently formally encoded in Check_initialized_correctly module in bench/bench.ml.