function resolve( error, results ) {
			if ( flg ) {
				// Prevent further processing:
				return;
			}
			if ( error ) {
				flg = true;
				return clbk( error );
			}
			out[ i ] = results;
			clbk();
		}
Callback invoked once a provided function finishes.

@private
@param {*} [error] - error
@param {*} [results] - results
@returns {void}