PropCheck.StateM.postcondition

You're seeing just the callback postcondition, go back to PropCheck.StateM module for more information.
Link to this callback

postcondition(s, call, res)

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Specs

postcondition(s :: dynamic_state(), call :: symbolic_call(), res :: term()) ::
  boolean()

Specifies the postcondition that should hold about the result res of performing call, given the dynamic state s of the abstract state machine prior to command execution.

This function is called during runtime, this is why the state is dynamic.