theorem Th21:
  a>=0 implies 1 -Root a = a
proof
  assume
A1: a>=0;
  a |^ 1 = a GeoSeq.(0+1) by Def1
    .= a GeoSeq.0 * a by Th3
    .= 1*a by Th3
    .= a;
  hence thesis by A1,Th19;
end;
