Evaluate a BXT
•	If the root is nil, return 0
•	If the root is a number, return its value
•	evaluate left subtree to get lvalue
•	evaluate right subtree to get the rvalue
•	use the root operator on rvalue and lvalue
•	Binary Expression trees can be easily built from prefix expressions only.
•	remove the head of the prefix list and put it in the root
•	if root is an operator
•	build the left subtree using remaining prefix expression (you may not exhaust the expression)
•	build the right subtree using the remaining prefix expression (you may not exhaust the expression)
•	Draw the binary expression tree for the following prefix expression and evaluate the tree.
•	+ * + - / 1 2 4 * 3 2 + 4 5 + 6 / + 3 3 3
•	The answer is 30.5
•	Creating BXTree ADT
•	constructor, destructor
•	void parse(ifstream& in)
•	void build()
•	double evaluate()
•	Does it need to be a template? No. The type of the nodes is obvious “Token”
class BXTree: public BinaryTree<Token>
{
	Queue<Token> q;
	public:
	void parse(ifstream& in);
	void build();
	double evaluate();
};

class Token
{
	friend class BXTree;
	char a[28];
	public:
	bool IsOperator();
	double value();
	friend istream& operator>>(istream&,Token&);
};