theorem Th17:
  for f,g being Assign of BASSModel(R,BASSIGN) holds s |= f 'or' g
  iff (s|= f or s|= g)
proof
  let f,g be Assign of BASSModel(R,BASSIGN);
  s |= f 'or' g iff not s|= 'not'(f) '&' 'not'(g) by Th12;
  then s |= f 'or' g iff not s|= 'not'(f) or not s|= 'not'(g) by Th13;
  hence thesis by Th12;
end;
