Essentially, `plasp`’s output format consists of [state variables](#variables) that are modified by [actions](#actions) if their preconditions are fulfilled.
Variables reference [entities](#constants-objects) that are affected by the actions.
As with PDDL, the objective is to achieve a specific [goal](#goal) starting from an [initial state](#initial-state) by executing a sequence of actions.
`plasp` recognizes and declares advanced features used by the input problem, such as [conditional effects](#actions), [mutex groups](#mutex-groups) and [axiom rules](#axiom-rules) (currently only SAS).
[Variables](#variables), [constants](#constants-objects), and [objects](#constants-objects) may be typed. Types are only available with PDDL and if typing is enabled.
Variables are named after the PDDL predicate signatures, `variable(<PDDL predicate>).`
Each variable contains exactly two values (one `true`, one `false`) of the form `value(<variable>, <bool>)`.
Note that with PDDL, values contain the corresponding variables as the first argument to make the format consistent with the multi-valued variables obtained with SAS input.
### Derived Variables
```prolog
% declares a <derivedvariable>
derivedVariable(derivedVariable(<name>)).
% adds a <value> to the domain of a <derivedvariable>
contains(<derivedvariable>, <value>).
```
Derived variables are introduced whenever the translator needs to use [derived predicates](#derived-predicates).
When the preconditions of a [derived predicate](#derived-predicates) are met, the corresponding derived variable (named after the derived predicate) is set as an effect.
Derived variables are analogously defined to common variables and may also be referenced in preconditions, conditions of conditional effects, and goal descriptions, just as variables.
In contrast to common variables, derived variables *are not subject to inertia rules.*
In other words, derived variables are computed for each time step separately, and reset when going to the next time step (their values don’t automatically carry over to the next time step).
After applying an action, variables get new values according to the action's postconditions.
Actions may have *conditional effects*, that is, certain postconditions are only applied if additional conditions are satisfied.
For this reason, each conditional effect is uniquely identified with a predicate `effect(<number>)` as the second argument of the `postcondition` facts.
The conditions of conditional effects are given by additional `precondition` facts that take the respective `effect(<number>)` predicates as the first argument.
Unconditional effects are identified with `effect(unconditional)`.
Conditional effects are currently only supported with SAS input problems.
Derived predicates are introduced by the translator when there are nested expressions or disjunctions in action preconditions, conditions of conditional effects, or goal descriptions.
Derived predicates operate on [derived variables](#derived-variables) of the same name.
Like actions, derived predicates must satisfy preconditions in order for their effect to be applicable.
The effect of all derived predicates is to set the corresponding [derived variables](#derived-variables) to `true` or `false`.
In contrast to actions, however, derived predicates specify whether their preconditions are to be interpreted as a *conjunction* (`type(and)`) or as a *disjunction* (`type(or)`).
Encoding authors need to ensure that derived predicate preconditions are interpreted in the correct way.
The goal specifies all assignments of [variables](#variables) and [derived variables](#derived-variables) that have to be fulfilled after executing the action sequence.
