Multiple 'ON DELETE CASCADE' constraints for identical foreign key

Question

Scenario

Imagine a chat server with conversations C and multiple posts P in each conversation. A user can reply to a post. This data should be stored as reply := (original_post_id, replyer_post_id).

To mimic this I have the tables P (posts) and R (replies):

CREATE TABLE P (
    id BIGINT NOT NULL,
    -- more data ...
)

CREATE TABLE R (
    orig_id BIGINT NOT NULL,
    repl_id BIGINT NOT NULL,

    FOREIGN KEY orig_id REFERENCES P(id),
    FOREIGN KEY repl_id REFERENCES P(id),
)

---

Problem

If I want to automaically delete all associated R-entries upon deletion inside P, I would change the last two lines of R's definition to:

    ...
    FOREIGN KEY orig_id REFERENCES P(id) ON DELETE CASCADE,
    FOREIGN KEY repl_id REFERENCES P(id) ON DELETE CASCADE,
)

However, I recieve the following error:

Introducing FOREIGN KEY constraint [...] on table [...] may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.

---

Question

Why exactly am I recieving this error? Technically it is not a cycle as neither R.orig_id nor R.repl_id are referenced by any other table (thus possbibly creating a cycle). I furthermore cannot see why multiple cascade paths should be an issue, as the deletion procedure would be as follows:

• Delete an entry from P with P.id = 1234
• Delete any entry from R with R.repl_id = 1234 or R.orig_id = 1234

Should I somehow constrain the columns R.repl_id and R.orig_id not to be equal?