reserve X for non empty UNITSTR;
reserve a, b for Real;
reserve x, y for Point of X;
reserve X for RealUnitarySpace;
reserve x, y, z, u, v for Point of X;

theorem Th28:
  0 <= ||.x.||
proof
  0 <= x .|. x by Def2;
  hence thesis by SQUARE_1:def 2;
end;
