function next( error, result ) {
			if ( error ) {
				return done( error );
			}
			i -= 1;
			if ( i < 0 ) {
				return done( null, result );
			}
			f[ i ]( result, next );
		}
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}