Using variables defined by piecewise functions

Hello,
I need to define a variable X_t given by
X_t = 0 if A_t < B_t,
and
X_t = A_t - B_t otherwise.
Is there a way of doing this in Dynare?
Many thanks,
Sébastien
I need to define a variable X_t given by
X_t = 0 if A_t < B_t,
and
X_t = A_t - B_t otherwise.
Is there a way of doing this in Dynare?
Many thanks,
Sébastien