function composeAsync() {
	var nFuncs;
	var f;
	var i;
	nFuncs = arguments.length;
	if ( nFuncs < 2 ) {
		throw new Error( 'insufficient arguments. Must provide multiple functions to compose.' );
	}
	f = new Array( nFuncs );
	for ( i = 0; i < nFuncs; i++ ) {
		f[ i ] = arguments[ i ];
		if ( !isFunction( f[ i ] ) ) {
			throw new TypeError( format( 'invalid argument. All arguments must be functions. Value: `%s`.', f[ i ] ) );
		}
	}
	return composite;

	/**
	* Composite function.
	*
	* @private
	* @param {...*} args - arguments
	* @param {Callback} done - callback to invoke after invoking all functions
	*/
	function composite() {
		var done;
		var args;
		var i;

		// Cache the callback function:
		done = arguments[ arguments.length-1 ];

		// Copy arguments which should be provided to the first invoked function...
		args = new Array( arguments.length-1 );
		for ( i = 0; i < args.length; i++ ) {
			args[ i ] = arguments[ i ];
		}
		// Append the callback an invoked function should call upon completion:
		args.push( next );

		// Start invoking provided functions:
		i = nFuncs - 1;
		f[ i ].apply( null, args );

		/**
		* Callback invoked upon completion of a composed function.
		*
		* @private
		* @param {(Error|null)} error - error object
		* @param {*} result - result to pass to next function
		* @returns {void}
		*/
		function next( error, result ) {
			if ( error ) {
				return done( error );
			}
			i -= 1;
			if ( i < 0 ) {
				return done( null, result );
			}
			f[ i ]( result, next );
		}
	}
}
Function composition.

@param {...Function} f - functions to compose
@throws {Error} must provide more than one argument
@throws {TypeError} must provide functions
@returns {Function} composite function

@example
function a( x, next ) {
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        next( null, 2*x );
    }
}

function b( x, next ) {
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        next( null, x+3 );
    }
}

function c( x, next ) {
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        next( null, x/5 );
    }
}

var f = composeAsync( c, b, a );

function done( error, result ) {
    if ( error ) {
        throw error;
    }
    console.log( result );
    // => 3
}

f( 6, done );