let combinePredecessors stmt ~old (new_:t) =
let new_state = computeFirstPredecessor stmt new_ in
let is_new, new_state = BuildPdg.test_and_merge_states old new_state in
if is_new then Some new_state
else
begin
(if !debug
then P.debug "fix point reached for stmt %d" stmt.sid);
None
end