Everything on the tube is drawn by SYMELEC, Heinz Lemke's 1972 PIXIE program, run from his listing. Your mouse is the light pen.

• The + in the middle is the tracking cross. It follows the pen. How it does that is Tracking.
• The letters around it are a radial menu, the earliest known one.
• The column on the right is the command menu. IN, SC, CA, RE and RO act on a selected element, so on an empty picture they do nothing.

Draw a line. Press on the cross and drag slowly; too fast and the cross is left behind.

1. Tap S, upper left of the cross. It turns into F: an element is open.
2. For a straight line tap RU on the right; the default, HV, draws in steps.
3. Drag. Let go and drag again for the next corner.
4. Tap F to finish.

Tap, don't hold. SYMELEC reads a menu item still under the pen about half a second later as a second tap, so a held S opens the element and closes it again. A light pen was touched to the glass and taken away; the page turns a click into the same short touch. The story is in the Bug journal, "S would not stay F".

A lit line the pen passes over catches the cross, so carry it around the picture, not across it.

Demo reboots and lets a scripted pen draw a house, a sun, a tree and a circuit. Every stroke is SYMELEC's; the script only moves the pen. The picture then nearly fills memory: one or two more elements and SYMELEC starts over, blank.

The readout at the top right is the pen position, read from SYMELEC's own variables at 5641 and 5640. Memory, source view, shows the 1972 listing with 👉 on the line the PDP-7 is running.

The three words this page patches, and why, are in the Bug journal. How 128 pages of printout became the file the page loads is the Transcription report.