function createBenchmark( len ) {
	var str;
	var i;

	str = '';
	for ( i = 0; i < len-1; i++ ) {
		if ( randu() < 0.5 ) {
			str += '0';
		} else {
			str += '1';
		}
	}
	return benchmark;

	/**
	* Benchmark function.
	*
	* @private
	* @param {Benchmark} b - benchmark instance
	*/
	function benchmark( b ) {
		var bool;
		var i;

		b.tic();
		for ( i = 0; i < b.iterations; i++ ) {
			bool = isBinaryString( str + (i%10) );
			if ( typeof bool !== 'boolean' ) {
				b.fail( 'should return a boolean' );
			}
		}
		b.toc();
		if ( isBoolean( bool ) ) {
			b.pass( 'benchmark finished' );
		} else {
			b.fail( 'should return a boolean' );
		}
		b.end();
	}
}
Creates a benchmark function.

@private
@param {PositiveInteger} len - string length
@returns {Function} benchmark function