diff --git a/playground/weak_constraints/greenfirst.py b/playground/weak_constraints/greenfirst.py
new file mode 100644
index 00000000..50645905
--- /dev/null
+++ b/playground/weak_constraints/greenfirst.py
@@ -0,0 +1,63 @@
+#! /usr/bin/env python
+# encoding: utf-8
+
+from waflib import Task, Runner
+
+# amount of tasks to wait before trying to mark tasks as done
+# see Runner.py for details
+# setting the gap to a low value may seriously degrade performance
+Runner.GAP = 1
+
+# shrinking sets of dependencies provided to know quickly which yelow tasks are ready
+reverse_map = {}
+
+old = Task.set_file_constraints
+def green_first(lst):
+	# weak order constraint based on lexicographic order:
+	# green before pink before yellow
+	lst.sort(cmp=lambda x, y: cmp(x.__class__.__name__, y.__class__.__name__))
+
+	# call the previous method to order the tasks by file dependencies
+	old(lst)
+
+	# shrinking sets preparation
+	for tsk in lst:
+		for k in tsk.run_after:
+			try:
+				reverse_map[k].add(tsk)
+			except KeyError:
+				reverse_map[k] = set([tsk])
+Task.set_file_constraints = green_first
+
+def get_out(self):
+	# this is called whenever a task has finished executing
+	tsk = self.prev_get_out()
+
+	for x in reverse_map.get(tsk, []):
+		# remote this task from the dependencies of other tasks
+		# this is a minor optimization in general
+		# but here we use this to know which tasks are ready to run
+		x.run_after.remove(tsk)
+
+	if tsk.__class__.__name__ == 'green':
+		# whenever a green task is done it may be time to put
+		# one or more yellow tasks in front
+		# some additional optimization can be performed if exactly one
+		# yellow task can be added to avoid whole list traversal
+		def extract_yellow(lst):
+			# return the yellow tasks that we can run immediately
+			front = []
+			lst.reverse()
+			for tsk in lst:
+				if tsk.__class__.__name__ == 'yellow' and not tsk.run_after:
+					#print("found one yellow task to run first")
+					lst.remove(tsk)
+					front.append(tsk)
+			lst.reverse()
+			return front
+		# this sets the order again
+		self.outstanding = extract_yellow(self.outstanding) + extract_yellow(self.frozen) + self.outstanding
+	return tsk
+Runner.Parallel.prev_get_out = Runner.Parallel.get_out
+Runner.Parallel.get_out      = get_out
+
diff --git a/playground/weak_constraints/wscript b/playground/weak_constraints/wscript
new file mode 100644
index 00000000..b0921892
--- /dev/null
+++ b/playground/weak_constraints/wscript
@@ -0,0 +1,40 @@
+#! /usr/bin/env python
+# encoding: utf-8
+
+"""
+Illustrates how to force yellow tasks to be executed very soon after the green ones
+"""
+
+import time
+
+def options(ctx):
+	ctx.load('parallel_debug')
+
+def configure(ctx):
+	ctx.load('parallel_debug')
+
+def build(ctx):
+
+	ctx.load('greenfirst', tooldir='.')
+
+	ctx.jobs = 4
+	def touch(tsk):
+		tsk.outputs[0].write('')
+		if tsk.__class__.__name__ == 'green':
+			time.sleep(0.2)
+		elif tsk.__class__.__name__ == 'yellow':
+			time.sleep(3)
+		else:
+			time.sleep(0.3)
+
+	for x in range(40):
+		ctx(rule=touch, always=True, name='pink', color='PINK', target='pink%s.txt' % x)
+
+	for x in range(5):
+		lst = []
+		for y in range(60):
+			name = 'green%s_%s.txt' % (x, y)
+			lst.append(name)
+			ctx(rule=touch, always=True, target=name, name='green', color='GREEN')
+		ctx(rule=touch, always=True, source=lst, name='yellow', color='YELLOW', target='yellow%s.txt' % x)
+