Undo, and the toolbar in three groups

Undo takes back one's own latest mark, by the button, Ctrl+Z or two
quick taps of the eraser; the toolbar's drawing tools, the page and
view, and the session stand in three groups that do not move when the
tool changes; the buttons are a little smaller.

docs/guest.md

@@ -48,10 +48,11 @@ place in it waits for the host to return.
 
 ## The screen
 
-Across the top is the toolbar, one row holding everything: the tools
-and their settings, the view, the host's page buttons, your notebook,
-and **Session**, which opens a window with the participants and the
-invitation.  Below it the page has the whole width of the window.  Every button
+Across the top is the toolbar, one row holding everything in three
+groups: on the left the tools, their settings and **Undo**; in the
+middle the page and the view; on the right the host's page buttons,
+your notebook, and **Session**, which opens a window with the
+participants and the invitation.  Below it the page has the whole width of the window.  Every button
 explains itself: rest the pointer on it for two seconds and its
 explanation appears beneath it.
 
@@ -117,7 +118,24 @@ and to the next ones.
 **Eraser** removes your own writing.  Move it across a stroke and the
 whole stroke goes; across a text block and the block goes.  It removes
 nothing of anyone else's, and nobody else's eraser can touch yours.  An
-erased stroke is gone from the page for everyone.
+erased stroke is gone from the page for everyone, until you undo the
+erasure.
+
+### Undo
+
+**Undo**, the last button of the drawing tools, takes back your latest mark: a
+stroke, what one movement of the eraser removed, a text block as you
+last typed or moved it, a Hide rectangle made, moved or removed.  Press
+it again for the mark before that, and so on back to the beginning of
+the page.  Ctrl+Z (Command+Z on a Mac) does the same, and so do two
+quick taps of the Eraser on empty paper, away from anything the eraser
+would remove; a tap that erases something is never half of such a
+double tap.  Undo acts only on your own marks, since nobody can change
+anyone else's, and only on marks made on this page since you opened it:
+after a reload of the tab, or on a page loaded from a notebook, there is
+nothing to undo although the marks are there.  The button is grey when
+there is nothing to undo.  An image cannot be undone; put it back with
+**Image** if a tap of the eraser removed it by mistake.
 
 ### The view
 

docs/host.md

@@ -195,7 +195,8 @@ anyone included, is covered in white, marked by blue corners that tell
 the students something is there.  Use it to cover a solution until the
 student has tried, or a part of the exercise for later.  Rest the pen on
 it to move it, near a corner to resize it; a tap of the eraser removes
-it and shows what was covered.  Nothing under a Hide is lost, and a
+it and shows what was covered, and **Undo** takes back any of these as
+it does your writing.  Nothing under a Hide is lost, and a
 student cannot uncover it by hiding your writing on their screen: a Hide
 belongs to the page, not to your writing.  A Hide is white in an
 exported picture and in the page file as other programs see it.

docs/implementation.md

@@ -151,6 +151,17 @@ layer from that participant.  A client that reconnects after a dropped
 connection sends its whole layer again, so that nothing drawn meanwhile
 is lost.
 
+Undo is personal, since nobody can change another's layer, and is a
+stack of one's own gestures kept in the browser tab for the page being
+written: a stroke; the strokes and text blocks one movement of the
+eraser removed, kept whole; a text block as it was before an editing or
+a move; a Hide rectangle as it was before it was made, moved or removed.
+Undoing sends the ordinary messages, except that erased strokes come
+back by a `stroke_restore` message carrying them whole.  Images are not
+in the stack, because their data is not kept.  The stack is cleared when
+the page changes and is not in the page file, so a reloaded tab starts
+with nothing to undo.
+
 ## The page file
 
 A notebook is a directory.  The page being written is `Current.svg` in
@@ -264,7 +275,10 @@ by which the page grows (`page_model.js`), the tooltip delay
 slack, `pen_input.js`), the reach of an image's corner (24 pixels,
 `renderer.js`), the smallest an image can be scaled to (1 cm,
 `page_model.js`), how often a dragged image's place is sent (40 ms,
-`session.js`) and the points of a stroke (25 ms, `session.js`), the colour and width of a text rectangle's frame
+`session.js`) and the points of a stroke (25 ms, `session.js`), the
+interval of the eraser's double tap (400 ms, `DOUBLE_TAP_MS` in
+`main.js`) and how many gestures can be undone (100, `UNDO_LIMIT` in
+`session.js`), the colour and width of a text rectangle's frame
 (`page_file.js`) and whether the frame is shown whole or as corners on
 screen (`TEXT_FRAME_STYLE` in `renderer.js`; the file and the export
 always hold the whole frame).