Index: src/libcfa/concurrency/threads
===================================================================
--- src/libcfa/concurrency/threads	(revision 0e76cf4f08013d765530952465f1547e8fd011e2)
+++ src/libcfa/concurrency/threads	(revision 9129a848d87bdbb3434b3db7693cba174b561aae)
@@ -17,8 +17,13 @@
 #define __THREADS_H__
 
+#include <stdbool.h>
 
 struct coroutine {
-      int blarg;
+      coroutine* last;
+      const char* name;
+      bool notHalted;
 };
+
+void ?{}(coroutine* this);
 
 trait coroutine_t(dtype T) {
@@ -26,6 +31,5 @@
 };
 
-forall(dtype T | coroutine_t(T))
-void suspend(T* cor);
+void suspend(void);
 
 forall(dtype T | coroutine_t(T))
Index: src/libcfa/concurrency/threads.c
===================================================================
--- src/libcfa/concurrency/threads.c	(revision 0e76cf4f08013d765530952465f1547e8fd011e2)
+++ src/libcfa/concurrency/threads.c	(revision 9129a848d87bdbb3434b3db7693cba174b561aae)
@@ -1,11 +1,54 @@
 #include "threads"
+#include "assert"
 
-forall(dtype T | coroutine_t(T))
-void suspend(T* cor) {
+#include <stddef.h>
 
+#include <fstream>
+ 
+static coroutine main_coroutine;
+static coroutine* current_coroutine = &main_coroutine;
+
+void ctxSwitchDirect(void* src, void* dst) {
+	current_coroutine = dst;
+}
+
+coroutine* this_coroutine() {
+	return current_coroutine;
+}
+
+void ?{}(coroutine* this)
+{
+	this->last = NULL;
+      this->name = "A Coroutine";
+      this->notHalted = true;
+}
+
+void suspend() {
+      coroutine* src = this_coroutine();		// optimization
+
+	assertf( src->last == (coroutine*)0, 
+		"Attempt to suspend coroutine %.256s (%p) that has never been resumed.\n"
+		"Possible cause is a suspend executed in a member called by a coroutine user rather than by the coroutine main.",
+		src->name, src );
+	assertf( src->last->notHalted, 
+		"Attempt by coroutine %.256s (%p) to suspend back to terminated coroutine %.256s (%p).\n"
+		"Possible cause is terminated coroutine's main routine has already returned.",
+		src->name, src, src->last->name, src->last );
+
+	ctxSwitchDirect( src, src->last );
 }
 
 forall(dtype T | coroutine_t(T))
 void resume(T* cor) {
+	coroutine* src = this_coroutine();		// optimization
+	coroutine* dst = this_coroutine(cor);
 
+	if ( src != dst ) {				// not resuming self ?
+		assertf( dst->notHalted , 
+			"Attempt by coroutine %.256s (%p) to resume terminated coroutine %.256s (%p).\n"
+			"Possible cause is terminated coroutine's main routine has already returned.",
+			src->name, src, dst->name, dst );
+		dst->last = src;					// set last resumer
+	} // if
+	ctxSwitchDirect( src, dst );				// always done for performance testing
 }