+It will be helpful to know if an itemset has been "completed" or not,
+particularly for LALR where itemsets get merged, at which point they
+need to be consider for completion again. So a `completed` flag is needed.
+
+For correct handling of `TK_newline` when parsing, we will need to
+know which states (itemsets) can occur at the start of a line, so we
+will record a `starts_line` flag too whenever DOT is at the start of a
+`line_like` symbol.
+
+Finally, for handling `TK_out` we need to know whether productions in the
+current state started *before* the most recent indent. A state
+doesn't usually keep details of individual productions, so we need to
+add one extra detail. `min_prefix` is the smallest non-zero number of
+symbols *before* DOT in any production in an itemset. This will allow
+us to determine if the the most recent indent is sufficiently recent
+to cancel it against a `TK_out`. If it was seen longer ago than the
+`min_prefix`, and if the current state cannot be reduced, then the
+indented section must have ended in the middle of a syntactic unit, so
+an error must be signaled.
+