Add record, union concepts

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diff --git a/examples/records-concept.lisp b/examples/records-concept.lisp
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+; Records are named tuples with an identifier. `record!` is a builtin function
+; that generates functions for creating, reading and modifying a record.
+; In type system terms, records are intersections.
+
+; `record!` can only be called in the top-level context.
+
+(record! Ns/User
+    name
+    phone-number)
+
+; Generates the following functions:
+
+(let! user
+    (Ns/User/create "John" "010-123-456"))
+    ; user ~> (Ns/User name:"John" phone-number:"010-123-456")
+
+(Ns/User/get-name user) ; => "John"
+(Ns/User/get-phone-number user) ; => "010-123-456"
+
+(Ns/User/set-name "Rick" user)
+    ; => (Ns/User name:"Rick" phone-number:"010-123-456")
+(Ns/User/set-phone-number unit user)
+    ; => (Ns/User name:"Rick" phone-number:<unit>)
+
+
+
+
+; Unions are unions of records with an identifier. `union!` is a builtin function
+; that generates functions for manipulating the records in the union.
+
+; `union!` can only be called in the top-level context.
+
+(union! Ns/Maybe
+    (just value)
+    (nothing))
+
+; Generates the following functions:
+
+(let! m1 (Ns/Maybe/just 10)) ; m1 ~> (Ns/Maybe/Just value:10)
+(let! m2 (Ns/Maybe/nothing)) ; m2 ~> (Ns/Maybe/Nothing)
+
+(Ns/Maybe/Just/get-value m1) ; => 10
+
+
+
+
+; The builtin function `kind` returns the identifier as a string for any record value
+
+(kind m1) ; => "Ns/Maybe/Just"
+(kind m2) ; => "Ns/Maybe/Nothing"
+(kind user) ; => "Ns/User"