Help adding memory buttons/commands in the calculator

[joelpub]

What should they do? memo suggests that there is a place to take a memo, some kind of storage location. What do you want to happen when memo_add() is called?

Dean, there are three buttons:
MS: save number to memory
M+: sum the memory saved number to the number on the screen
M-: subtract the memory saved number from the number on the screen
Like a real calculator.

#### Buttons I don't know how to make it work
Button(width=4, text='Ms', relief='flat', command=self.memo_add).place(x=360, y=50) # must save to memory
Button(width=4, text='M+', relief='flat', command=self.memo_sum).place(x=360, y=90) # must sum to the screen value
Button(width=4, text='M-', relief='flat', command=self.memo_sub).place(x=360, y=130) # must subtract from the screen value

#### COMMANDS I NEED HELP WITH
def memo_add(self): # must save number to memory
def memo_sum(self): # must sum the saved number to the screen value
def memo_sub(self): # must subtract from the screen value