def test_pstr1():
s1 = pstr("123")
s2 = "1"
assert s1 - s2 == "23"
def test_pstr4():
s1 = pstr("123")
s2 = "456"
assert s1 - s2 == "123"
def test_pstr5():
s1 = pstr("")
s2 = "456"
assert s1 - s2 == ""
def test_pstr2():
s1 = pstr("123")
s2 = "123"
assert s1 - s2 == ""
