Stylus that does not allow subtraction

We noticed an issue when using the Semantic Grid stylus.

*width total-width * ((((gutter-width + column-width ) * x) - gutter-width) / _gridsystem-width)-correction

      

        
        
        
      

    

processes something along lines (reads different numbers)

  *width: 89.5833333333333% -correction;

      

        
        
        
      

    

So, I decided to do an experiment. On startup, stylus -i

I found that subtraction also doesn't work (at all)

To make sure I wrote this in my file

x=10
y=20
.something
    font-size y-x

      

        
        
        
      

    

which processes

.something {
  font-size: y-x;
}

      

        
        
        
      

    

I've gone crazy? Did I misinterpret the statements with the stylus? Pending results here, I can open a ticket to github. I found another person with a percentage question that highlights that the operator -

is not working as expected. However, this doesn't seem like my problem.

TL; DR: Basic math without units doesn't seem to work