[LONGITUDINAL]
input =  {a,b}

EQUATION:
lp1 = a-b*t
lp2 = a-b*t/2

DEFINITION:
y = {type           = categorical, 
     categories     = {1,2,3},
     logit(P(y<=1)) = lp1,
     logit(P(y<=2)) = lp2}
