Solved by KFrank in post #2 
              
              
                Hi Dryden! 

You therefore want four classes, “background”, “class 10”, “class 12”, and 
“class 13.”  You should then label them with with the four integer class 
labels 0, 1, 2, 3. 
Set up a translation table that maps the integers {0, 1, 2, ..., 17, 18} 
to your desired smaller set of class labels…