how to mark all unread messages read in mutt?

[Ranjan Maitra via Mutt-users]

Hi,

Sorry for a very basic question, but is there a key-binding or command sequence 
to mark all unread messages in mutt?

I tried the following, following suggestions online:

Declared: 

macro index <F3> "<C-v><enter>" "mark tagged as read"

Then, I went and did:

T~U  Hit Return: Hit F3,

but nothing happens: mutt just mocks me. 

Is there a way to actually make this work?

Many thanks again, and best wishes,
Ranjan