PropCheck.FSM.precondition

You're seeing just the callback precondition, go back to PropCheck.FSM module for more information.
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precondition(from, target, state_data, call)

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Specs

precondition(
  from :: state_name(),
  target :: state_name(),
  state_data :: state_data(),
  call :: symbolic_call()
) :: boolean()

Similar to PropCheck.StateM.precondition/2.

Specifies the precondition that should hold about state_data so that call can be included in the command sequence. In case precondition doesn't hold, a new transition is chosen using the appropriate state_name/1 generator.

It is possible for more than one transitions to be triggered by the same symbolic call and lead to different target states. In this case, at most one of the target states may have a true precondition. Otherwise, PropEr will not be able to detect which transition was chosen and an exception will be raised.