slice

Allocates the next weight of this scope's window to a sub-scope.

weight is a fraction in 0..1 of this scope's window. The returned scope maps its own 0..1 reports into that window, and its slices nest further. A non-null phase labels the slice and is reported immediately at its start; an unnamed slice inherits the current label. A weight beyond the remaining window is clamped, and weights should sum to at most 1.