--- nand.py-
+++ nand.py
@@ -3,9 +3,10 @@
 import sys
 
 class Obj:
-	def __init__(self, parent, name):
+	def __init__(self, parent, name, latency=None):
 		self.parent = parent
 		self.name = name
+		self.latency = latency
 		if parent:
 			setattr(parent, name, self)
 
@@ -17,6 +18,9 @@
 		pa = self.parent.get_name() + '.' if self.parent else ''
 		return pa + me
 
+	def enque(self, f, args=() ):
+		sched.enque(self.latency, f, args)
+
 class Pin(Obj):
 	def __init__(self, parent, name, direc='IO', v=None):
 		Obj.__init__(self, parent, name)
@@ -37,28 +41,39 @@
 		if 'O' in self.direc and self.conn:
 			self.conn.set(v)
 		if 'I' in self.direc and self.parent and hasattr(self.parent, 'update'):
-			sched.enque(None, self.parent.update)
+			self.enque(self.parent.update)
 
 class Foo(Obj):
 	def __init__(self, parent, latency=None):
-		Obj.__init__(self, parent, 'foo')
-		self.latency = latency
+		Obj.__init__(self, parent, 'foo', latency)
 		self.v = 0
 
 		Pin(self, 'inp', 'I')
 		Pin(self, 'out', 'O')
 
-		sched.enque( latency, self.add, (0, ) )
+		self.enque( self.add, (0, ) )
 
 	def add(self, a):
 		self.v += a
 		self.dbgout( 'v={}'.format(self.v) )
 		if self.v < 55:
-			sched.enque( self.latency, self.add, (a+1,) )
+			self.enque( self.add, (a+1,) )
 
 	def update(self):
 		v = not self.inp.v
-		sched.enque( self.latency, self.out.set, (v,) )
+		self.enque( self.out.set, (v,) )
+
+class NAND(Obj):
+	def __init__(self, parent, name='nand', latency=10):
+		Obj.__init__(self, parent, name, latency)
+		Pin(self, 'out', 'O')
+		Pin(self, 'inp_a', 'I')
+		Pin(self, 'inp_b', 'I')
+
+	def update(self):
+		self.dbgout('update')
+		v = not ( self.inp_a.v and self.inp_b.v )
+		self.enque( self.out.set, (v,) )
 
 class Sched:
 	def __init__(self):
@@ -70,6 +85,10 @@
 		if latency:
 			tm = tm + latency
 		t = (tm, f, args)
+
+		if self.exist_same(t):
+			return
+
 		n = len(self.que)
 		while n > 0:
 			(tm_n, _, _) = self.que[ n - 1 ]
@@ -78,6 +97,9 @@
 			n -= 1
 		self.que.insert(n, t)
 
+	def exist_same(self, t):
+		return next( ( True for a in self.que if a == t ), False )
+
 	def main_loop(self):
 		while len(self.que) > 0:
 			(tm, f, args) = self.que.pop(0)
@@ -108,6 +130,13 @@
 	o.slow.foo.out.connect( o.slow_12.foo.inp )
 	sched.enque( 200, o.foo.out.set, (True,) )
 
+	NAND(o)
+	sched.enque( 300, o.nand.inp_a.set, (False,) )
+	sched.enque( 300, o.nand.inp_b.set, (False,) )
+	sched.enque( 320, o.nand.inp_a.set, (True,) )
+	sched.enque( 340, o.nand.inp_b.set, (True,) )
+	sched.enque( 360, o.nand.inp_a.set, (False,) )
+
 	sched.main_loop()
 
 # EOF