Ceylon: overriding equals and hash

	/**
	Best practises for implementing Equals and Hash methods
	Source: http://www.artima.com/lejava/articles/equality.html
	------------------------------------------------------------
	Common pitfalls when overriding equals:
	- Defining equals with the wrong signature.
	- Changing equals without also changing hashCode.
	- Defining equals in terms of mutable fields.
	- Failing to define equals as an equivalence relation.
	The contract of the equals method in Object specifies that equals must
	implement an equivalence relation on non-null objects:
	It is reflexive:
	for any non-null value x, the expression x.equals(x) should return true.
	It is symmetric:
	for any non-null values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
	It is transitive:
	for any non-null values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true,
	then x.equals(z) should return true.
	It is consistent:
	for any non-null values x and y, multiple invocations of x.equals(y) should consistently return true
	or consistently return false, provided no information used in equals comparisons on
	the objects is modified. For any non-null value x, x.equals(null) should return false.
	**/
	shared abstract class Colour(shared String name) of yellow | orange | green {
	shared actual Boolean equals(Object that) {
	if (is Colour that) {
	return that.canEqual(this) && name == that.name;
	}
	return false;
	}
	shared actual default Integer hash => name.hash;
	shared default Boolean canEqual(Object that) => that is Colour;
	}
	shared object yellow extends Colour("yellow") {}
	shared object orange extends Colour("orange") {}
	shared object green extends Colour("green") {}
	shared interface Coloured {
	shared formal Colour colour;
	}
	shared class Fruit(name) {
	shared String name;
	shared actual default Boolean equals(Object that) {
	if (is Fruit that) {
	return that.canEqual(this) && name == that.name;
	}
	return false;
	}
	shared actual default Integer hash => name.hash;
	shared default Boolean canEqual(Object that) => that is Fruit;
	}
	shared class ColouredFruit(String name, colour) extends Fruit(name) satisfies Coloured {
	shared actual Colour colour;
	shared actual Boolean equals(Object that) {
	if (is ColouredFruit that) {
	return that.canEqual(this) && name.equals(that.name) && colour.equals(that.colour);
	}
	return false;
	}
	shared actual default Integer hash => name.hash + colour.hash;
	shared actual default Boolean canEqual(Object that) => that is ColouredFruit;
	}
	/**
	comparefruits() prints
	banana1 equal to banana2 = false
	banana2 equal to banana1 = false
	**/
	void comparefruits() {
	Fruit banana1 = Fruit("banana");
	ColouredFruit banana2 = ColouredFruit("banana", yellow);
	print("banana1 equal to banana2 = ``banana1.equals(banana2)``");
	print("banana2 equal to banana1 = ``banana2.equals(banana1)``");
	}

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