Defines a function of x, y and z.
s is a string which contains the expression for calculating the values of the function at given (x, y, z) locations. It must have a ";" at the end, and can contain the predefined tokens "x", "y" and "z", the operators +, -, *, / and ^ , the functions of the C math library sin, cos, tan, sinh, cosh, tanh, asin, acos, atan, atan2, exp, log, log10, pow, sqrt, ceil, floor and fmod, and the names of variables. v is a list of name and variable pairs (like for example ("pi", PI)), which define under which names variables can be referenced in the formula.
s must be a string node, and v a typeless node to which the above mentioned (name, variable) pairs are connected.