typedef unsigned(1) token;

place red token: 4#1;
place black token: 3#1;

trans rr 
in { place red: 2#1;}
out { place black: 1;};

trans bb 
in { place black: 2#1;}
out { place black: 1;};

trans rb
in { place red: 1;
      place black: 1;}
out { place red: 1;};