spectre bsource for current stimuli

Hi,

I am a new spectre user. 

I need to implement a current stimuli which starts at a time tstart >0, has a linear rise till a certain time trise until the current reaches a peak Ipeak and then has an exponential fall with time constant taufall.

I understand that I can implement an exponential function : Ixy (x y) bsource I=Ipeak*exp(-($time - trise)/taufall)

But how can join the linear rise part to this? 

This is not a periodic source.

I would also like to add, when I try the similar function with an inhouse SPICE simulator, I have the possibility of using an if else statement in behavioral source, which functions like below :

if time <= trise + tstart, then 

     if time < tstart

       Ixy = 0

    else

     Ixy … defining a pwl function

else

I=Ipeak*exp(-($time - trise)/taufall)

Is something similar possible in spectre?