Calc is the canonical calculator grammar, as found in so many documents
discussing context free grammars. 

Call "calc" as usual ... enter simple arithmetic strings as usual ...
then at EOF the program will traverse your input expressions and print
them out in various forms.  Which form? By default, it will print them
as infix. But you can invoke calc with a single command line argument
to ask for prefix or postfix output as well ... e.g. 
	calc prefix
or
	calc postfix
or
	calc infix

