xt0 (ProcessComb NDC _ pl pr) = next0OrChild pl [1] `S.union` next0OrChild pr [2]
    where next0OrChild p' pos = if blocking p' then
                                pos <.> next0 p'
                               else S.singleton pos
n
next0 (ProcessComb NDC _ pl pr) = next0OrChild pl [1] `S.union` next0OrChild pr [2]
    where next0OrChild p' pos = if blocking p' then
                                pos <.> next0 p'
                               else S.singleton pos
next0 (ProcessComb NDC _ pl pr) = next0OrChild pl [1] `S.union` next0OrChild pr [2]
    where next0OrChild p' pos = if blocking p' then
                                pos <.> next0 p'
                               else S.singleton pos