Introduce the Handler API.

reactor/API/src/jetbrains/mps/logic/reactor/program/Handler.java

@@ -0,0 +1,14 @@
+package jetbrains.mps.logic.reactor.program;
+
+/**
+ * @author Fedor Isakov
+ */
+public abstract class Handler {
+
+    public abstract String name();
+
+    public abstract ConstraintSymbol primarySymbol();
+
+    public abstract Iterable<Rule> rules();
+
+}

reactor/API/src/jetbrains/mps/logic/reactor/program/Program.java

@@ -14,7 +14,13 @@ public abstract class Program {
 
   public abstract Iterable<PredicateSymbol> predicateSymbols();
 
+  /**
+   * @deprecated use handlers().foreach{ yieldAll it.rules() }
+   * @return
+   */
+  @Deprecated
   public abstract Iterable<Rule> rules();
 
+  public abstract Iterable<Handler> handlers();
 
 }

reactor/Test/src/program/MemProgram.kt

@@ -88,6 +88,8 @@ class MemProgram(val name: String, val myRules : List<Rule>, val registry: Const
         registry.predicateSymbols()
 
     override fun rules(): Iterable<Rule> = unmodifiableCollection(myRules)
+
+    override fun handlers(): MutableIterable<Handler> = TODO()
 }