function parallel( done ) {
		if ( !isFunction( done ) ) {
			throw new TypeError( format( 'invalid argument. Callback argument must be a function. Value: `%s`.', done ) );
		}
		return limit( fcns, opts, clbk );

		/**
		* Callback invoked upon completion.
		*
		* @private
		* @param {*} [error] - error
		* @param {Array} [out] - output array
		* @returns {void}
		*/
		function clbk( error, out ) {
			if ( error ) {
				return done( error );
			}
			done( null, out );
		}
	}
Executes a set of functions in parallel and returns an array of results.

@private
@param {Callback} done - function to invoke upon completion
@throws {TypeError} must provide a function
@returns {void}