reserve X for non empty CUNITSTR;
reserve a, b for Complex;
reserve x, y for Point of X;
reserve X for ComplexUnitarySpace;
reserve x, y, z, u, v for Point of X;

theorem Th21:
  (x-y).|.z = x.|.z - y.|.z
proof
  (x - y) .|. z = x .|. z + (-y) .|. z by Def11
    .= x .|. z + ( - y .|. z ) by Th18;
  hence thesis;
end;
