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 Th10:
  (-x) .|. (-y) = x .|. y
proof
  (-x) .|. (-y) = - x .|. (-y) by Th8
    .= - ( - x .|. y ) by Th8;
  hence thesis;
end;
