theorem Th28:
  x\(x``) is positive Element of X
proof
  (x\((x`)`))` = x`\(((x`)`)`) by BCIALG_1:9
    .=x`\x` by BCIALG_1:8
    .=0.X by BCIALG_1:def 5;
  then 0.X<=x\((x`)`);
  hence thesis by Def2;
end;
