Bob Mueller has an unreleased Trump letter about firing James B. Comey. Here’s why that’s big.

.. Which brings us to that letter. According to The Post reporters, who were briefed on its contents, it doesn’t dwell on Russia, but it does express concern Comey wouldn’t say publicly Trump himself wasn’t under investigation.

.. The letter was drafted with top White House adviser Stephen Miller, a former Senate aide to now-Attorney General Jeff Sessions. Miller has been among the most controversial and ideologically extreme members of the White House staff

.. We also know the letter was multiple pages — suggesting plenty to pick apart — and aides cautioned Trump against at least some of its contents. The Times reports White House counsel Don McGahn believed the letter was “problematic.”