public synchronized void setOperator(String op) {
		if (op.equals(ADD)) {
			this.op = ADD;
		} else if (op.equals(SUBTRACT)) {
			this.op = SUBTRACT;
		} else if (op.equals(MULTIPLY)) {
			this.op = MULTIPLY;
		} else if (op.equals(DIVIDE)) {
			this.op = DIVIDE;
		} else {
			System.out.println("Undefined Operator in EquationBTreeNode.");
		}
	}