Fence a record once per node, refuse a column reading its own record, and keep a bare reference independent as an operand

README.md

@@ -126,12 +126,14 @@ renders nothing by `Fake`, and is compiled only so the tree's fences still run.
 The columns are the point, and their facts stay together: two columns that
 reference one category — `{/currency.code}` and `{/currency.symbol}` — share one
 draw of it, so the record is internally consistent. That one draw is also why two
-columns may not read overlapping reference paths — `{/cat.a}` beside `{/cat.a.b}`
-is refused, naming the fields to write instead, exactly as
-[One draw, one spelling](#one-draw-one-spelling) refuses the pair inside a single
-format. A field hold, transform or operand ties fields together within one column
-as always (see [Correlated fields](#correlated-fields) and
-[Decisions](#decisions)).
+columns may not read overlapping reference *paths* — `{/cat.a}` beside
+`{/cat.a.b}` is refused, naming the fields to write instead, as
+[One draw, one spelling](#one-draw-one-spelling) refuses that pair inside a single
+format. A column may not reference the record it belongs to either: `{/users.first}`
+inside `users` would contradict the `first` column beside it, so put a value two
+columns share in its own category and reference that. A field hold, transform or
+operand ties fields together within one column as always (see
+[Correlated fields](#correlated-fields) and [Decisions](#decisions)).
 
 ## Library