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In step 2 you must estimate the function of the surface from the variables X and Y. To do so, 21^st Century Project is able to handle more complex data structures like vectors and arrays. X is used as index and the Z values are stored in an array. In the following calculation unit the whole vectors are stored in an array using Y as index to create a two dimensional array. You can use 21^st Century Project in the way of APL. This function representation will be exported in point 4.