Got CSS Preprocessor version 1.0.6 published.
This fixes a crashing problem with the minus operator (the following: "field: -;" crashes version 1.0.5 and earlier.) But that was not the point, just a side effect of writing many more tests.
The new system supports the @return at-command which means user defined functions work.
I wrote the necessary tests to check the system defined external functions. They all get checked. The result is that a couple were moved to internal functions so they would work (unique_id() and percentage() which cannot otherwise be implemented) and I added one function deg2rad() because the hue() function returns angles in degrees.
The result is an even more complete preprocessor!