Vb中自定义函数的返回值包含自身，这种情况如何判断？求解释

Question

Private Sub Command1__Click()
Print test("abc")
End Sub

Function test(s As String) As String
For i = 1 To 3
test = Mid(s, i, 1) + test
Next i
End Function

输出的结果是“cba”,变成字符串拼接的操作了，不是很理解里面的原理。求解释