I heard a story that someone giving a talk at IAS only used M's on the chalkboard (lowercase, uppercase, doublestroke, script, etx) before he was told to cut it

I heard a story that someone giving a talk at IAS only used M's on the chalkboard (lowercase, uppercase, doublestroke, script, etx) before he was told to cut it out. I can't remember who it was if anyone else heard the story.