[LONGITUDINAL]
input =  {a,b}

EQUATION:
lp0 = a-b*t
lp1 = a-b*t/2

DEFINITION:
y = {type           = categorical, 
     categories     = {0,1,2},
     logit(P(y<=0)) = lp0,
     logit(P(y<=1)) = lp1}