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+# Subtyping of record types
+
+It's a serious limitation of SRFI 9 that it doesn't allow creating
+subtypes with additional fields.  This is an invaluable strategy for
+representing a hierarchy of types, which are ubiquitious in real life
+and thus in programming.
+
+Sadly, this brings with it some significant complications if records
+are to be initialized with default values to ensure invariants.  The
+R6RS solves this with an incredibly sophisticated system, which we
+might need to adopt.  (Search for "protocol" and "record-constructor
+descriptor" in the R6RS.)
+
+However, we may be able to get away with a simpler approach...
+
+UNDER CONSTRUCTION
+
+Are constructor protocols really that important?  Consider that all we
+can do is add additional fields in the subtype.  What if we separated
+allocation from initialization:
+
+```scheme
+(define-record r1
+  (parent #f)
+  (fields a b))
+
+(define (init-r1! r a b)
+  (set-r1-a! a)
+  (set-r1-b! b))
+
+
+(define-record r2
+  (parent r1)
+  (fields c d))
+
+(define (init-r2! r a b c d)
+  (init-r1! r a b)
+  (set-r2-c! c)
+  (set-r2-d! d))
+
+
+(define-record r3
+  (parent r2)
+  (fields e f))
+
+(define (init-r3! r a b c d e f)
+  (init-r2! r a b c d)
+  (set-r3-e! e)
+  (set-r3-f! f))
+
+
+(define r (make-r3))
+(init-r3! r 1 2 3 4 5 6)
+```