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+
+The cdr-code is a 4-bit field of a 32-bit cons cell.
+It is treated as an on-page indicator bit and a 3-bit scaled offset.
+
+Certain combinations are treated specially.
+
+
+on-page| offset	| interpretation
+-------|--------|----------------------------------------------------------------------------
+   1   |    0	| CDR is NIL
+   1   |  1 - 7	| CDR is at 2*offset (counted in 32-bit cells) on same page
+   0   |    0	| CDR is indirect, CDR(car)
+   0   |  1 - 7	| CDR is not a cons cell but is in the car of cell at 2*offset on same page
+-------|--------|----------------------------------------------------------------------------
+
+