theorem Th78:
  for X be set, S be right_zeroed non empty addLoopStr
  for p,q be Series of X, S
  for V be set st vars p c= V & vars q c= V holds
    vars (p+q) c= V
proof
  let X be set, S be right_zeroed non empty addLoopStr;
  let p,q be Series of X, S;
  let V be set;
  assume vars p c= V & vars q c= V;
  then vars (p+q) c= vars p \/ vars q c= V by Th41,XBOOLE_1:8;
  hence thesis;
end;
