Instructions

	The statements that may be entered are as follows:
<statement>	::= <assign> ";" | <command>
<assign>	::= "cell" "=" <expression>
<expression>	::= <term> <plusopterm>
<plusopterm>	::= "" | ("+" | "-") <term> <plusopterm>
<term>		::= <factor> <mulopfactor>
<mulopfactor>	::= "" | ("*" | "/") <factor> <mulopfactor>
<factor>	::= "number" | "cell" | "(" <expression? ")" 
			| "sum" "(" <range> ")" | "prod" "(" <range> ")" 
<range>		::=	"cell" ".." "cell" 
<command>	::= "eval" | "print" | "dump" | "done" 

"cell" 		is a single letter followed by an integer (both in legal range): e.g. a4.
"number"	is an unsigned integer.  To get -5 use 0-5.  There is no unary minus. Sorry.



 example commands
  A6 = 5;		// The letter names the column and the integer the row. 
  B8 = A6 + 1;		// also elaborate expressions with operators +-*/ and parens.
  A9 = sum(B1..C4);	// also prod(range)  where range == cell..cell

The accept button accepts commands as you enter them, but does not evaluate the sheet. 
The clear button clears the command input area.
The evaluate button evaluates the entire sheet. 

Commands are entered into the command field at the bottom of the frame that initially
says Welcome.  This text field may be cleared at any time with the Clear button.  Enter
a command into the command box and then push the Accept Button.  The sheet will not be
updated until you push the Evaluate button.  Note that all commands are terminated by
semicolons.  Only 26 columns may be used (letters a-z). 

IMPORTANT NOTE: A recursive command (e.g. A0=A0+1) is illegal. There is no check for it 
in this program however.  Your program will eventually crash if you try it. It is also 
possible to write mutually recursive sets of commands.  These are just as illegal.   
 

 Exercises:  (In approximate increasing order of difficulty )
	Add a unary minus.  Use ~ for this operator, not -.  ~5 means minus 5. 
	Add the modulo operator to the language. Use % for this operator.  12 % 5 should
		 yield 2. 
	Give the program a prettier interface. 
	Add an additional range function. Choose a good one.  
	Add an exponentiation operator. Use ^ for this operator.  2^5 should yield 32.
		The precidence level of ^ should be higher than that of * and /, but lower than 
		parens. Note that if you permit 2^5^3, that it should group from the right 
		as 2^(5^3).  The included operators all group from the left within precidence 
		levels. 	
	Add a logic operator similar to the C++ conditional expression operator ?:.  To the 
		left of the ? you need a comparison like a4<3.  To the right you need two 
		expressions separated by the colon.  
	What improvements can you make that will increase the total code size by no more than 
		one page?

Explanation of classes

Class Spreadsheet is derived from applet.  It is a view-controller class.  It does not have
any functionality beyond managing the views and the buttons/fields.  

Class SpreadsheetModel is the actual spreadsheet.  It uses a class SpreadsheetCell for
the cells in the rectangular array.  Class SemanticRecord is used to hold information
in spreadsheet programs.  A Semantic record is either a value or a cell reference. Each
cell may represent a simple value or a spreadsheet program.   

A spreadsheet program is a vector of pairs of integers and semantic records. It represents
an expression written in postfix.  In other words.  expression a+b is represented in the
program as " a b +"  The operands are either constants or other cells.  

A spreadsheet program is evaluated by pushing operands onto an evaluation stack and 
popping them when an operator needs to be evaluated.  The result of the operation is then
pushed back on the stack.  

A range of cells is represented internally by listing all of its members.  Thus programs
that use ranges may be quite long.  They are maintained in fully expanded form.