add pydel_of_py_is_safe(x)

... which is true if `pydel!(Py(x))` is safe (i.e. `Py(x)` definitely returns a new value).

Or perhaps `!ispy(x)` is enough??

Start by reading the existing definitions and documentation for pydel!, Py(x), and ispy(x). Clarify whether the requested predicate should guarantee that Py(x) returns a new value, then check the project's existing API tests for a suitable place to specify the behavior. Done means the API design is decided and its behavior is covered by tests.

索引モデルが issue の本文から書いたものです。