[NTG-context] 2DOWN imposition gives blank pages

[Sanjoy Mahajan]

Taco wrote:
> Don't you need:
> 
>    \setuppapersize[A5,landscape][A4,landscape]
>    \setuparranging[2DOWN,rotated]
> 

The following produces a first page with p.1 on the right half, but a
blank 2nd page:

\setuppapersize[A5,landscape][A4,landscape]
\setuparranging[2DOWN,rotated]
\starttext
\input tufte
\stoptext

That looks okay, except for the blank 2nd page.

But shouldn't it also work to impose two A5's on one landscape A4?
Here's a few more puzzles related to that.  If they are doing what
they are supposed to, let me know -- then I need to understand the
imposition system more.

\setuppapersize[A5][A4,landscape]
\setuparranging[2UP]
\starttext
\input tufte
\page
\input tufte
\stoptext

produces a two-page document:

p.1: |   | 1 |  (i.e. blank left half, and original p.1 on the right half)
p.2: | 2 |   |  (i.e. original p.2 on left half, blank right half)

Shouldn't it produce

p.1: | 1 | 2 |

Here's my reasoning.  Two A5's (with their long directions vertical)
fit side by side to make one A4 in landscape orientation with text
running down along the short direction, which looks like 2-column text
done the hard way.  So, for a two-page original document, 2UP
arranging should produce either

p.1: | 1 | 2 |

or

p.1: | 2 | 1 |


So in case I've confused the directions of 2UP and 2DOWN, I also tried:

\setuppapersize[A5][A4,landscape]
\setuparranging[2DOWN]
\starttext
\input tufte
\page
\input tufte
\stoptext

It produces:

p.1: |   |   |
p.2: | 2 |   |

The original p.1 disappeared, which I don't understand.  Also I don't
understand why it results in two output pages.

But this hack:

\setuppapersize[A5][A4,landscape]
\setuparranging[2UP]
\starttext
\null\page
\input tufte
\page
\input tufte
\stoptext

produces the following arrangement, which is close to what I expect:

p.1: |   | 1 |  (where the original p.1 is blank except for the page num)
p.2: | 2 | 3 |

Once I understand what's going on, I'll wikify!

-Sanjoy