[LONGITUDINAL]
input = {k1, k2, f0, g0, tau}

PK:
depot(type=1, target=f, Tlag=tau)
depot(type=2, target=g, p=f/2)

EQUATION:
t0  = 0
f_0 = f0
g_0 = g0
ddt_f = -k1*f + k2*g 
ddt_g =  k1*f - k2*g 
