UPDATE 1910 GMT: Rah-e-Sabz has published the full text of the Karroubi letter to Rafsanjani.
BBC Persian is reporting that opposition figure Mehdi Karroubi has made a pointed intervention with a letter to former President Hashemi Rafsanjani, in Rafsanjani's capacity as the head of the Assembly of Experts.
Karroubi's letter, sent to Rafsanjani just before this week's bi-annual Assembly meeting, called on the Assembly to exercise its powers to "monitor the functions and institutions under the auspices of Iran's Supreme Leader". Karroubi cited problems such as "a lack of independence of the judiciary and courts", the interference of the Islamic Revolution Guards Corps and Basij militia in political issues, and the IRGC's expanded involvement in Iran's economy.
And, in an even more provocative challenge, Karroubi pointed to the Assembly's powers, under the Iranian Constitution, to remove the Supreme Leader if he becomes incapable of carrying out his supervisory role.
The un-subtle message: if Ayatollah Khamenei won't step up and take control of the Revolutionary Guard and security forces, ensuring the independence of the courts and upholding the Constitution, then Rafsanjani and the Assembly need to do so.
Mir Hossein Mousavi's website Kalemeh had reported on Wednesday that Karroubi had written a letter to Rafsanjani "to be released in forthcoming days".