/// <summary>
		/// Initializes a new instance of the Lexer class with the specified
		/// TextReader to lex.
		/// </summary>
		/// <param name="source">A TextReader to lex.</param>
		public Lexer(TextReader source)
		{
			// token queue
			tokens = new TokenList();

			// read the file contents
			reader = new CharReader(source);
		}
		/// <summary>
		/// Returns a parsed DPL instance.
		/// </summary>
		/// <param name="reader">The TextReader value to be parsed.</param>
		/// <returns>A parsed Program instance.</returns>
		private IExpr ParseExpr(TextReader reader)
		{
			IExpr result=null;
			Lexer lexer = new Lexer(reader);
			tokens = lexer.Lex();

			if (tokens.Peek().Type == TokenTypes.EQUAL)
			{
				tokens.Extract();		// skip over the equal
				curToken = tokens.Extract();	// set up the first token
				MatchExprAndOr(out result);	// start with lowest precedence and work up
			}

			if (curToken.Type != TokenTypes.EOF)
				throw new ParserException("End of expression expected." + "  At column " + Convert.ToString(curToken.StartCol));

			return result;
		}