+
+ If R1 points to a tagged object it points either to
+ * A constructor.
+ * A function with arity <= TAG_MASK.
+ In both cases the right thing to do is to return.
+ Note: it is rather lucky that we can use the tag bits to do this
+ for both objects. Maybe it points to a brittle design?
+
+ Indirections can contain tagged pointers, so their tag is checked.