theorem Th201:
  0**A c= {0}
proof
  let a;
  assume a in 0**A;
  then consider c1,c2 such that
A1: a = c1*c2 and
A2: c1 in {0} and
  c2 in A;
  c1 = 0 by A2,TARSKI:def 1;
  hence thesis by A1,TARSKI:def 1;
end;
