store a list of functions and their inverse
[]=define_func2inv(func,invfunc)
func = a string vector of functions
iunvfunc = a string vector of the inverse of these functions
nothing: string vectors are stored in a database
define_func2inv(['log';'exp';'sin';'cos';'log10';'sqrt'],['exp';'log';'asin';'acos';'exp10';'square']) // Example taken from define_func2inv_d. Stores the list of some basic functions and their inverse (for instance the inverse of function log is defined as function ... exp!)