Add examples for Logical models

Examples/Logicals/logical-example.fsh

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+// @Name: Logical Example
+// @Description: An example of Logical resources and how to use them
+
+// A 'Logical' is neither a resource nor a datatype. It can be thought of
+// as a data class.
+Logical: AnimalAbilities
+* hasLungs 0..1 boolean "Does the animal have lungs?"
+* canSwim 0..1 boolean "Can the animal swim?"
+
+// A Logical can be used in place of a "complex datatype"
+Logical: Animal
+* name 1..1 string "The animal's name"
+* age 0..1 decimal "The animal's age in years"
+* traits 1..1 AnimalAbilities "The animal's abilities"
+
+// A Logical can be a Parent (inheritance)
+Logical: Pet
+Parent: Animal
+* isVaccinated 0..1 boolean "Is the pet vaccinated?"
+
+// A Logical can be like a Resource, but you have to add an id element and the 
+// #can-be-target Characteristic, so that References work.
+Logical: PetAsResourceLikeObject
+Parent: Animal
+Characteristics: #can-be-target
+* id 0..1 string "The id"
+* isVaccinated 0..1 boolean "Is the pet vaccinated?"
+
+// Example of Reference from one Logical to another
+Logical: Aquarium
+* members 0..* Reference(PetAsResourceLikeObject) "The pets inside the aquarium"
+
+// Instance examples:
+Instance: ExamplePetPhil
+InstanceOf: PetAsResourceLikeObject
+* name = "Phil"
+* age = 2
+* traits.canSwim = true
+
+Instance: ExamplePetClaire
+InstanceOf: PetAsResourceLikeObject
+* name = "Claire"
+* age = 3
+* traits.canSwim = false
+
+Instance: ExampleAquarium
+InstanceOf: Aquarium
+* members[0] = Reference(ExamplePetPhil)
+* members[1] = Reference(ExamplePetClaire)