Declare marks: cursors that stay before inserted text

`state from: mark in note` is a cursor that stays before text inserted
at its position; `cursor in` goes after it, as a caret does. Everything
a cursor takes, a mark takes too. The notes example's mark is now one,
and its reference model agrees: typing at the mark leaves it in place.

1 files changed, 2 insertions, 1 deletions

diff --git a/tests/check.py b/tests/check.py
index c4484c4..7f94235 100644
--- a/tests/check.py
+++ b/tests/check.py
@@ -445,7 +445,8 @@ class NotesModel:
     def insert(self, text):
         p = self.at
         self.note = self.note[:p] + text + self.note[p:]
-        self.at, self.mark = [c + len(text) if c >= p else c for c in (self.at, self.mark)]
+        self.at = self.at + len(text) if self.at >= p else self.at
+        self.mark = self.mark + len(text) if self.mark > p else self.mark  # a mark stays at the insertion point
 
     def remove(self, a, b):
         self.note = self.note[:a] + self.note[b:]