theorem Th37:
  {a}"" = {a"}
proof
  let z;
  hereby
    assume z in {a}"";
    then consider c such that
A1: z = c" and
A2: c in {a};
    c = a by A2,TARSKI:def 1;
    hence z in {a"} by A1,TARSKI:def 1;
  end;
  assume z in {a"};
  then
A3: z = a" by TARSKI:def 1;
  a in COMPLEX & a in {a} by TARSKI:def 1,XCMPLX_0:def 2;
  hence thesis by A3;
end;
