FAST-+/FIXNUM and similar should accept unboxed arguments in interests
of representation selection. Problem: inter-TN dependencies.
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+#14
+The derived type of (/ (THE (DOUBLE-FLOAT (0D0)) X) (THE (DOUBLE-FLOAT
+1D0) Y)) is (DOUBLE-FLOAT 0.0d0). While it might be reasonable, it is
+better to derive (DOUBLE-FLOAT (-0.0d0)).
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