xt (ProcessComb NDC _ pl pr) = nextOrChild pl [1] `S.union` nextOrChild pr [2]
    where nextOrChild p' pos = if blocking p' then
                                pos <.> next p'
                               else S.singleton pos
n
next (ProcessComb NDC _ pl pr) = nextOrChild pl [1] `S.union` nextOrChild pr [2]
    where nextOrChild p' pos = if blocking p' then
                                pos <.> next p'
                               else S.singleton pos
next (ProcessComb NDC _ pl pr) = nextOrChild pl [1] `S.union` nextOrChild pr [2]
    where nextOrChild p' pos = if blocking p' then
                                pos <.> next p'
                               else S.singleton pos