refactor: improve Trigger type hierarchy

jakta-api/src/main/kotlin/it/unibo/jakta/beliefs/BeliefBase.kt

@@ -3,7 +3,7 @@ package it.unibo.jakta.beliefs
 import it.unibo.jakta.beliefs.BeliefBase.Update
 
 /** A BDI Agent's collection of [Belief]s */
-interface BeliefBase<Query : Any, Belief, SelfType : BeliefBase<Query, Belief, SelfType>> : Collection<Belief> {
+interface BeliefBase<in Query : Any, out Belief, SelfType : BeliefBase<Query, Belief, SelfType>> : Collection<Belief> {
 
     /**
      * Performs unification between [B] and values in this [BeliefBase]

jakta-api/src/main/kotlin/it/unibo/jakta/context/AgentContext.kt

@@ -12,10 +12,13 @@ import it.unibo.jakta.plans.PlanLibrary
 /**
  * The Context is the actual state of a BDI Agent's structures.
  */
-interface AgentContext<B : Belief<*>, C : BeliefBase<B, C>, T : Trigger<*>> {
+interface AgentContext<Query, Belief, BB, T> where
+      Query : Any,
+      BB : BeliefBase<Query, Belief, BB>,
+      T : Trigger<*> {
 
     /** [BeliefBase] of the BDI Agent */
-    val beliefBase: C
+    val beliefBase: BB
 
     /**
      * The collection of [Event] that the BDI Agent reacts on.

jakta-api/src/main/kotlin/it/unibo/jakta/events/Trigger.kt

@@ -1,52 +1,51 @@
 package it.unibo.jakta.events
 
-import it.unibo.jakta.beliefs.Belief
 import it.unibo.jakta.beliefs.BeliefBase
 
 /** [Trigger] denotes the change that took place for the [Event] generation. */
-sealed interface Trigger<out X> {
-    val value: X
+sealed interface Trigger<out Query> {
+    val value: Query
 }
 
-/** [Trigger] generated after a [Belief] addition (or removal) from the [BeliefBase]. */
-interface BeliefBaseRevision<out X : Belief<*>> : Trigger<X>
+sealed interface BeliefBaseRevision<out Query> : Trigger<Query> {
+    /** [Trigger] generated after a Belief addition to agent's [BeliefBase]. */
+    data class BeliefBaseAddition<out Query>(override val value: Query) : BeliefBaseRevision<Query> {
+        override fun toString(): String = "BeliefBaseAddition(value=$value)"
+    }
 
-/** [Trigger] generated after a [Belief] addition to agent's [BeliefBase]. */
-class BeliefBaseAddition<X : Belief<*>>(override val value: X) : BeliefBaseRevision<X> {
-    override fun toString(): String = "BeliefBaseAddition(value=$value)"
-}
-
-/** [Trigger] generated after a [Belief] removal from agent's [BeliefBase]. */
-data class BeliefBaseRemoval<X : Belief<*>>(override val value: X) : BeliefBaseRevision<X> {
-    override fun toString(): String = "BeliefBaseRemoval(value=$value)"
-}
+    /** [Trigger] generated after a Belief removal from agent's [BeliefBase]. */
+    data class BeliefBaseRemoval<out Query>(override val value: Query) : BeliefBaseRevision<Query> {
+        override fun toString(): String = "BeliefBaseRemoval(value=$value)"
+    }
 
-data class BeliefBaseUpdate<X : Belief<*>>(override val value: X) : BeliefBaseRevision<X> {
-    override fun toString(): String = "BeliefBaseUpdate(value=$value)"
+    data class BeliefBaseUpdate<out Query>(override val value: Query) : BeliefBaseRevision<Query> {
+        override fun toString(): String = "BeliefBaseUpdate(value=$value)"
+    }
 }
 
 /** [Trigger] of an event made by a [Test] Goal. */
-interface TestGoalTrigger<out X> : Trigger<X>
+sealed interface TestGoalTrigger<out Query> : Trigger<Query> {
+    /** [Trigger] generated after an invocation of a [Test] Goal. */
+    data class TestGoalInvocation<out Query>(override val value: Query) : TestGoalTrigger<Query> {
+        override fun toString(): String = "TestGoalInvocation(value=$value)"
+    }
 
-/** [Trigger] generated after an invocation of a [Test] Goal. */
-data class TestGoalInvocation<X>(override val value: X) : TestGoalTrigger<X> {
-    override fun toString(): String = "TestGoalInvocation(value=$value)"
-}
-
-/** [Trigger] generated after a failure of a [Test] Goal. */
-data class TestGoalFailure<X>(override val value: X) : TestGoalTrigger<X> {
-    override fun toString(): String = "TestGoalFailure(value=$value)"
+    /** [Trigger] generated after a failure of a [Test] Goal. */
+    data class TestGoalFailure<out Query>(override val value: Query) : TestGoalTrigger<Query> {
+        override fun toString(): String = "TestGoalFailure(value=$value)"
+    }
 }
 
 /** [Trigger] of an event made by a [Achieve] Goal. */
-interface AchievementGoalTrigger<out X> : Trigger<X>
+sealed interface AchievementGoalTrigger<out Query> : Trigger<Query> {
 
-/** [Trigger] generated after the invocation of a [Achieve] Goal. */
-data class AchievementGoalInvocation<X>(override val value: X) : AchievementGoalTrigger<X> {
-    override fun toString(): String = "AchievementGoalInvocation(value=$value)"
-}
+    /** [Trigger] generated after the invocation of a [Achieve] Goal. */
+    data class AchievementGoalInvocation<out Query>(override val value: Query) : AchievementGoalTrigger<Query> {
+        override fun toString(): String = "AchievementGoalInvocation(value=$value)"
+    }
 
-/** [Trigger] generated after the failure of a [Achieve] Goal. */
-data class AchievementGoalFailure<X>(override val value: X) : AchievementGoalTrigger<X> {
-    override fun toString(): String = "AchievementGoalFailure(value=$value)"
+    /** [Trigger] generated after the failure of a [Achieve] Goal. */
+    data class AchievementGoalFailure<out Query>(override val value: Query) : AchievementGoalTrigger<Query> {
+        override fun toString(): String = "AchievementGoalFailure(value=$value)"
+    }
 }