Compilation error in ceylon

tree.ceylon

abstract class Tree() {
  shared formal Integer evaluate();
}

class UnaryOperator(Tree branch, Integer f(Integer t)) extends Tree() {
	
	evaluate() => f(branch.evaluate());
}

class Constant(Integer constantValue) extends Tree() {
	evaluate() => constantValue;
}

class BinaryOperator(Tree left, Tree right, Integer f(Integer left, Integer right)) extends Tree() {
	evaluate() => f(left.evaluate(), right.evaluate());
}

class Plus(Tree left, Tree right) extends BinaryOperator(left, right, function(Integer left, Integer right) => left+right) {
	
}



void testIt() {
	value minus = function(Integer a) => -a;
	Tree t = UnaryOperator(Constant(2), minus);
	print(t.evaluate());
	
	Tree t2 = BinaryOperator(Constant(2), Constant(4), function(Integer left, Integer right) => left*right);
	print(t2.evaluate());
	
	// Work
	Tree t3 = Plus(Constant(2), Constant(3));
	print(t3.evaluate());
	
	// Doesn't work
	value t4 = Plus(Constant(2), Constant(3));
	print(t4.evaluate());
}