[Proj] Understanding 2D Helmert

[Andre Joost]

Dear list,

since PROJ 5.2 now has made its way into the OSGEO4W binaries, I try to 
understand the new features.

I started up with the 2D helmert transformation with this command line:

echo 1000 0 | proj +proj=helmert +convention=coordinate_frame +x=0 +y=0 
+s=1. +theta=-5

This should rotate a point with X=1000 Y=0 meters by theta=-5 around the 
origin.

But the result of "17.45 0" does not really make sense to me.
X=2000 results to a new X=34.91.

Can anybody shed a light on this?