'''21''' is a two player game, the game is played by choosing 
a number ('''1''', '''2''', or '''3''')  to be added to the ''running&nbsp;total''.

The game is won by the player whose chosen number causes the ''running&nbsp;total''
to reach ''exactly''&nbsp;'''21'''.

The ''running total'' starts at zero. 
One player will be the computer.
 
Players alternate supplying a number to be added to the ''running&nbsp;total''. 

This game is a variant of [https://en.wikipedia.org/wiki/Nim#The_21_game Nim].

;Task:
Write a computer program that will:
::* do the prompting (or provide a button menu), 
::* check for errors and display appropriate error messages, 
::* do the additions (add a chosen number to the ''running&nbsp;total''), 
::* display the ''running&nbsp;total'', 
::* provide a mechanism for the player to quit/exit/halt/stop/close the program,
::* issue a notification when there is a winner, and
::* determine who goes first (maybe a random or user choice, or can be specified when the game begins). 

;Related tasks:
::* [[Nim game]]
<br><br>
