Incremental development of Project 3
1. Create package that holds the lex function which was main routine in Project 2.
	- In parser, call lex and check that output is good. Hint: /dev/tty is the name of the terminal
2. Create object for list of lexemes.
	- Update lex to place lexemes in that container.
	- Test; make sure lexemes are placed properly and accessible as expected.
3. Create symbol table and update lex to record declarations (in dec part) 
	- Test; make sure symbol table looks right
		Print of symbol table should contain symbols and line numbers
4. Update lex to look them up (in program body)
	- Identifier declared twice in dec section
	- Identifier already declared in body section when reference is made.
		Test 
5. Write parser
	- Start with basic program and skeleton set of rules 

Note: Lexemes are stored for use during parsing. The symbol table is created and accessed during scanning.

Minimum test program:
program abc
dec
  x:int
begin
  Read(x)
end.

Productions involved: <program> <dec part> <list var> <type> <statement part> <statement> <input> <ident>

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Exceptions
- System exceptions can be caught...  when <System Error> follow with action
- Users may declare their own exceptions
	- Use raise to fire them
	- when still catches them

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Stack Example
- StackData is the abstract base class
- DataPtr is used to facilitate polymorphism

Use of 'null record' abstracts an object that will be made concrete in a subclass.

Stack example creates a linked list stack where each node has two pointers.
- a link
- a pointer to the data, which in this case is handled polymorphically
	* Data could be integer or string

Note: Adjust on the stack is a copy of the entire linked list

Freeing node is done by declaring a function as a new Ada.Unchecked_Deallocation( <Data>, <Pointer>)