I'm struggling with this really simple (vanilla plainTeX) trick, which I'm not able to crack...

How does one draw a vertical (i.e. perpendicular to the horizon) line that leads from the top (or top margin or as topmost as possible) of the current page to the current position (specifically the position of text at the time where the macro solving this problem is called)?

P. S. It may have unclear but the current position is meant including both the horizontal and vertical position. I. e.

```
Here I \myruler am...

Another paragraph...

```

would produce sth like

```
       |
       |
       |
       |
       |
Here I | am...

Another paragraph...

```