class Test

	var _c as Counter?

	def main is shared
		Test().run

	def inc(c as Counter)
		c.count = c.count + 1

	def run
		.testLocal
		.testClassVar

	def testLocal
		c as Counter?

		# the key here is that the calls to .inc() are accepted even
		# though its parameter type is Counter not Counter?

		if c
			.inc(c)
		assert c is nil

		c = Counter()

		if c
			.inc(c)
		assert c.count==1

		if c is not nil
			.inc(c)
		assert c.count==2

		if c<>nil
			.inc(c)
		assert c.count==3

	def testClassVar
		assert _c is nil

		if _c
			.inc(_c)

		_c = Counter()
		if _c
			.inc(_c)
		assert _c.count==1

		if _c is not nil
			.inc(_c)
		assert _c.count==2

		if _c<>nil
			.inc(_c)
		assert _c.count==3


class Counter

	var _count as int = 0

	pro count from var