function iterUnitspace( start, stop ) {
	var STOP;
	var iter;
	var curr;
	var FLG;
	if ( !isNumber( start ) || isnan( start ) ) {
		throw new TypeError( format( 'invalid argument. First argument must be a number. Value: `%s`.', start ) );
	}
	if ( arguments.length > 1 ) {
		if ( !isNumber( stop ) || isnan( stop ) ) {
			throw new TypeError( format( 'invalid argument. Second argument must be a number. Value: `%s`.', stop ) );
		}
		STOP = stop;
	} else {
		STOP = PINF;
	}
	curr = start - 1;

	// Create an iterator protocol-compliant object:
	iter = {};
	setReadOnly( iter, 'next', next );
	setReadOnly( iter, 'return', end );

	// If an environment supports `Symbol.iterator`, make the iterator iterable:
	if ( iteratorSymbol ) {
		setReadOnly( iter, iteratorSymbol, factory );
	}
	return iter;

	/**
	* Returns an iterator protocol-compliant object containing the next iterated value.
	*
	* @private
	* @returns {Object} iterator protocol-compliant object
	*/
	function next() {
		if ( FLG ) {
			return {
				'done': true
			};
		}
		curr += 1;
		if ( curr > STOP ) {
			FLG = true;
			return {
				'done': true
			};
		}
		return {
			'value': curr,
			'done': false
		};
	}

	/**
	* Finishes an iterator.
	*
	* @private
	* @param {*} [value] - value to return
	* @returns {Object} iterator protocol-compliant object
	*/
	function end( value ) {
		FLG = true;
		if ( arguments.length ) {
			return {
				'value': value,
				'done': true
			};
		}
		return {
			'done': true
		};
	}

	/**
	* Returns a new iterator.
	*
	* @private
	* @returns {Iterator} iterator
	*/
	function factory() {
		return iterUnitspace( start, STOP );
	}
}
Returns an iterator which returns numbers incremented by `1`.

@param {number} start - starting value (inclusive)
@param {number} [stop=+infinity] - stopping value
@throws {TypeError} first argument must be a number
@throws {TypeError} second argument must be a number
@returns {Iterator} iterator

@example
var iter = iterUnitspace( 0, 99 );

var v = iter.next().value;
// returns 0

v = iter.next().value;
// returns 1

v = iter.next().value;
// returns 2

// ...