diff --git a/makina-compiler/src/xyz/colinholzman/makina/State.kt b/makina-compiler/src/xyz/colinholzman/makina/State.kt
index 30468d8..58b5a82 100644
--- a/makina-compiler/src/xyz/colinholzman/makina/State.kt
+++ b/makina-compiler/src/xyz/colinholzman/makina/State.kt
@@ -127,6 +127,20 @@ class State(val id: String,
         }
     }
 
+    fun getInitialSubState(): State {
+        return subStates.find { it.initial } ?: subStates.first()
+    }
+
+    //gets a list of the sub states to enter if this is the target of a transition
+    fun getDefaultEntrySet(): List<State> {
+        return if (isLeafState()) {
+            emptyList()
+        } else {
+            val initialSubState = getInitialSubState()
+            listOf(initialSubState) + initialSubState.getDefaultEntrySet()
+        }
+    }
+
     override fun equals(other: Any?): Boolean {
         if (this === other) return true
         if (javaClass != other?.javaClass) return false
diff --git a/makina-compiler/src/xyz/colinholzman/makina/Transition.kt b/makina-compiler/src/xyz/colinholzman/makina/Transition.kt
index bab382b..3ea61c4 100644
--- a/makina-compiler/src/xyz/colinholzman/makina/Transition.kt
+++ b/makina-compiler/src/xyz/colinholzman/makina/Transition.kt
@@ -3,6 +3,10 @@ package xyz.colinholzman.makina
 import xyz.colinholzman.makina.State.Companion.getLCCA
 
 data class Transition(val source: State, val target: State) {
+    init {
+        if (!source.isLeafState())
+            throw RuntimeException("source must be a leaf state")
+    }
     fun getEntrySet(): List<State> {
         return (listOf(target) + target.getProperAncestors(listOf(source, target).getLCCA())).reversed()
     }
diff --git a/makina-compiler/test/xyz/colinholzman/makina/ExternalTransitionTest.kt b/makina-compiler/test/xyz/colinholzman/makina/ExternalTransitionTest.kt
new file mode 100644
index 0000000..9218f8a
--- /dev/null
+++ b/makina-compiler/test/xyz/colinholzman/makina/ExternalTransitionTest.kt
@@ -0,0 +1,46 @@
+package xyz.colinholzman.makina
+
+import org.junit.jupiter.api.Assertions
+import org.junit.jupiter.api.Test
+import java.io.File
+import java.util.concurrent.TimeUnit
+
+class ExternalTransitionTest {
+
+    val output = listOf(
+        "s1_entry",
+        "s1_s2_entry",
+        "s1_s2_e1_guard",
+        "s1_s2_exit",
+        "s1_exit",
+        "s1_entry",
+        "s1_s2_entry",
+        "s1_s2_e1_guard",
+        "s1_s2_exit",
+        "s1_exit",
+        "s1_entry",
+        "s1_s2_entry",
+        "s1_s2_e2_guard",
+        "s1_s2_exit",
+        "s1_s2_entry",
+        "s1_s2_e2_guard",
+        "s1_s2_exit",
+        "s1_s2_entry"
+    )
+    @Test
+    fun externalTransition() {
+        val makeClean = ProcessBuilder()
+                .directory(File("./../test/external_transition"))
+                .command("make", "clean").start()
+        makeClean.waitFor(10, TimeUnit.SECONDS)
+
+        val make = ProcessBuilder()
+                .directory(File("./../test/external_transition"))
+                .command("make").start()
+        make.waitFor(10, TimeUnit.SECONDS)
+
+        File("./../test/external_transition/out/test_output.txt").reader().use {
+            Assertions.assertEquals(output, it.readLines())
+        }
+    }
+}
\ No newline at end of file
diff --git a/test/external_transition/.gitignore b/test/external_transition/.gitignore
new file mode 100644
index 0000000..0f1f446
--- /dev/null
+++ b/test/external_transition/.gitignore
@@ -0,0 +1,2 @@
+out
+.vscode
\ No newline at end of file
diff --git a/test/external_transition/makefile b/test/external_transition/makefile
new file mode 100644
index 0000000..ad1ce61
--- /dev/null
+++ b/test/external_transition/makefile
@@ -0,0 +1,16 @@
+.PHONY: test
+test: out/test
+	./out/test
+
+out/test: test_driver.c out/test.c out/test.h out
+	gcc test_driver.c out/test.c -I "." -std=c99 -Wall -Wextra -Werror -o out/test
+	
+out/test.c:
+	java -jar ../../makina-compiler/out/artifacts/makina_compiler_jar/makina-compiler.jar test_source.mkna -o out
+
+out:
+	mkdir out
+
+.PHONY: clean
+clean:
+	rm -rf out
diff --git a/test/external_transition/test_driver.c b/test/external_transition/test_driver.c
new file mode 100644
index 0000000..36415c1
--- /dev/null
+++ b/test/external_transition/test_driver.c
@@ -0,0 +1,64 @@
+
+#include <stdio.h>
+#include "out/test.h"
+
+FILE *output;
+
+int s1_s2_entry(struct test *self, struct test_event *event) {
+    (void)self; (void)event;
+    fprintf(output, "s1_s2_entry\n");
+    return 0;
+}
+
+static int guard = 0;
+
+int s1_s2_e1_guard(struct test *self, struct test_event *event) {
+    (void)self; (void)event;
+    fprintf(output, "s1_s2_e1_guard\n");
+    return guard;
+}
+
+int s1_s2_e2_guard(struct test *self, struct test_event *event) {
+    (void)self; (void)event;
+    fprintf(output, "s1_s2_e2_guard\n");
+    return guard;
+}
+
+int s1_s2_e1_action(struct test *self, struct test_event *event) {
+    (void)self; (void)event;
+    fprintf(output, "s1_s2_e1_action\n");
+    return 0;
+}
+
+int s1_s2_exit(struct test *self, struct test_event *event) {
+    (void)self; (void)event;
+    fprintf(output, "s1_s2_exit\n");
+    return 0;
+}
+
+int s1_entry(struct test *self, struct test_event *event) {
+    (void)self; (void)event;
+    fprintf(output, "s1_entry\n");
+    return 0;
+}
+
+int s1_exit(struct test *self, struct test_event *event) {
+    (void)self; (void)event;
+    fprintf(output, "s1_exit\n");
+    return 0;
+}
+
+int main (void) {
+    output = fopen("out/test_output.txt", "w+");
+    struct test instance;
+    test_init(&instance);                                                       /*s1_entry, s1_s2_entry*/
+    test_process_event(&instance, &(struct test_event){.id = test_event_e1});   /*s1_s2_e1_guard, s1_s2_exit, s1_exit, s1_entry, s1_s2_entry*/
+    guard = 1;
+    test_process_event(&instance, &(struct test_event){.id = test_event_e1});   /*s1_s2_e1_guard, s1_s2_exit, s1_exit, s1_entry, s1_s2_entry*/
+    guard = 0;
+    test_process_event(&instance, &(struct test_event){.id = test_event_e2});   /*s1_s2_e2_guard, s1_s2_exit, s1_s2_entry*/
+    guard = 1;
+    test_process_event(&instance, &(struct test_event){.id = test_event_e2});   /*s1_s2_e2_guard, s1_s2_exit, s1_s2_entry*/
+    fclose(output);
+    return 0;
+}
\ No newline at end of file
diff --git a/test/external_transition/test_source.mkna b/test/external_transition/test_source.mkna
new file mode 100644
index 0000000..03d42fd
--- /dev/null
+++ b/test/external_transition/test_source.mkna
@@ -0,0 +1,14 @@
+machine test;
+
+state s1 {
+    on e1 -> s1;
+    on e2 -> s2;
+    entry s1_entry;
+    state s2 {
+        entry s1_s2_entry;
+        on e1 (s1_s2_e1_guard) -> s1;
+        on e2 (s1_s2_e2_guard) -> s2;
+        exit s1_s2_exit;
+    }
+    exit s1_exit;
+}