new file:   tree/Builder.java
	new file:   tree/Constant.java
	new file:   tree/Ex1.java
	new file:   tree/Ex2.java
	new file:   tree/Expression.java
	new file:   tree/Operation.java
	new file:   tree/Operator.java
	new file:   tree/ParsingException.java
	new file:   tree/Symbol.java
	new file:   tree/TestToString.java
	new file:   tree/UnknownVariableException.java
	new file:   tree/Variable.java