Trying to understand this: is the lambda symbol defined somewhere globally? How can they just use it Willy nilly as a generic Stand-in for a spread parameter type?
It's defined as
type λ<TA extends any[] = any[], TR = any> = (...args: TA) => TR;
There's a lot of interesting discussions around it as some math libraries like to use math symbols and greek letters and sometimes even emoji as variable/class names in JS and other developers have a gut reaction that they do not like it.