Function set 

pub fn set<T: Send + 'static, I: IntoIterator<Item = Handle<T>>>(
    iter: I,
) -> Set<T>

Available on crate feature coroutine only.

Create a set of handles, that can be joined concurrently.

Typically, you would spawn the tasks and collect the handles in a Vec, and convert it to a set with this function. Later, you can use Set::extend to add more handles.