In the example encoding, derived variables were accidentally included in
the inertia rule, as this only checked the position of the parameter.
This commit fixes this by only allowing common variables (and not
derived ones) within the inertia rule.
This change doesn’t affect derived variables, because these aren’t
affected by the “caused” predicate (the “caused” predicate requires a
variable to be in the postcondition of an action, and these may never be
derived variables).
The example encoding assumed that derived predicates weren’t necessary
in the final time step. This is, however, incorrect, as the goal may
have a precondition that requires derived predicates. In this case, the
derived variables must be evaluated for the final time step.
