yacts, yet another continuous time simulator, uses J+ to interpret scripts, defining systems of ordinary differential equations, which it solves. J+ is a functional non-imperative simulation language with lazy evaluation, based on the J programming language, a dialect of APL. A program in J+ is a collection of (possibly functional) J assignment statements, just like a set of formulae on a sheet of paper. The interpreter, implemented as a software library, knows the interdependencies between these formulae and is able to compute any of the defined quantities with minimal effort, keeping track of the values which were already computed. The driver program (such as yacts) may ask the interpreter to compute certain quantities and set the values for others (propagating the changes to dependents).