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return a mechanism and it's initialization step (may be a noop that returns a nil data blob and handle to the first "real" challenge step).
StepFunc implementations should never return a nil StepFunc result. This helps keep the logic in the SASL authticatee simpler: step functions are never nil. Mechanisms that end up an error state (for example, some decoding logic fails...) should return a StepFunc that represents an error state. Some mechanisms may be able to recover from such.