Subset by tag

[Enchufa2]

Now that activities can be tagged, this could be possible:

> x <- trajectory() |>
>   timeout(1, tag="one") |>
>   timeout(2, tag="two") |>
>   timeout(3, tag="three")
> x["two"]
trajectory: anonymous, 1 activities
{ Activity: Timeout      | [two] delay: 2 }