Hello, This is a question from a relative newbie in GiNaC. I'm running GiNaC 1.5, and have been able to wrap some of my own simple functions with the DECLARE and REGISTER macros. This works very smoothly. Great! However, now I want to make a next step, and I don't know how to proceed. The next new home-made function should take an array of expressions as an argument. Inside the new function, the array needs to be indexed by a second parameter to the new function. The new function is an interpolating function. I have tried regular arrays, lst, and matrix, but I can't find a way to pass the expression array to the evalf function, in such a way that it can be indexed by a second expression argument. Is it correct that indexing of a lst by an expression is not supported in ginac? Which approach would you recommend to construct an interpolating function, taking an array of expressions to be interpolated, and an expression as interpolation value? Can you give me an example evalf function declaration? Thanks in advance! Evert-Jan