The "in" operator allows testing whether a given value is an item of
a specified array or whether a given key is present in a specified
dictionary.


1. The `in` operator returns true if the given element is an item of
the specified array. Strict equality tests are performed.

-- Expect stdout --
[
	true,
	false,
	true,
	false,
	true,
	false,
	true,
	false
]
-- End --

-- Testcase --
{%
	let o = {};
	let a = [ o, {}, "", null, false ];

	printf("%.J\n", [
		o in a,
		{} in a,
		"" in a,
		"test" in a,
		null in a,
		[] in a,
		false in a,
		true in a
	]);
%}
-- End --

2. Strict equality when testing array membership should rule out implict
type coercion.

-- Expect stdout --
[
	true,
	false,
	false,
	false,
	true,
	false,
	false
]
-- End --

-- Testcase --
{%
	let a = [ "", true ];

	printf("%.J\n", [
		"" in a,
		0 in a,
		false in a,
		null in a,
		true in a,
		1 in a,
		1.0 in a
	]);
%}
-- End --

3. While there is the rule that `(NaN === NaN) == false`, testing for NaN
in a given array containing NaN should yield `true`.

-- Expect stdout --
[
	true
]
-- End --

-- Testcase --
{%
	let a = [ NaN ];

	printf("%.J\n", [
		NaN in a
	]);
%}
-- End --

4. When the `in` operator is applied to an object, it tests whether the given
string value is a key of the specified object. 

-- Expect stdout --
[
	true,
	true,
	true,
	false,
	false,
	false,
	false,
	false
]
-- End --

-- Testcase --
{%
	let o = { 
		"1": true,
		"test": false,
		"empty": null,
		"false": 0,
		"true": 1,
		"[ ]": "array",
		"{ }": "object"
	};

	printf("%.J\n", [
		"1" in o,
		"test" in o,
		"empty" in o,
		1 in o,        // not implicitly converted to "1"
		false in o,    // not implicitly converted to "false"
		true in o,     // not implicitly converted to "true"
		[] in o,       // not implicitly converted to "[ ]"
		{} in o        // not implicitly converted to "{ }"
	]);
%}
-- End --