Write a general purpose currency convertor program. The program is started
by giving it three program arguments: the name of the currency the user holds
and wishes to sell, the name of the currency the user wishes to buy, and the
conversion rate from the first currency to the other. For example, the application
might be given these arguments:
USDollar Euro 0.9428
to tell it that one US Dollar can buy 0.9428 of one Euro.
Once it is started, the application asks the user to type the amount of currency
the user wishes to sell. When the user submits this information, the application
computes the amount of currency purchased and displays the answer as two
proportionally sized coins:

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