Tkinter menu item underline syntax [RESOLVED]

Wed Jan 6 16:03:58 EST 2021

On 2021年01月06日, Rich Shepard <rshepard at appl-ecosys.com> wrote:
> On Wed, 6 Jan 2021, 2QdxY4RzWzUUiLuE at potatochowder.com wrote:
>>> For the sake of future generations who may run into this issue, can you
>> post the complete, correct call to file_menu.add_command?
>> This is the working version of the stanza I initially posted:
>> 	file_menu.add_command(
> label = 'New',
> command = self.callbacks['file->new', underline: 0],
> accelerator = 'Ctrl+N'
> )

I'm completely baffled by that. Can somebody explain how this
expression is evaluated?
 self.callbacks['file->new', underline: 0]
It appears that the dict callbacks is being accessed with the key of 
a tuple comprising a string and a slice.
Huh?