2.1 Callbacks

Solvers require as one argument a user function to work on which have to be provided by the user. These callbacks typically are of the form

f( x, params)
...
return result
Please note that this function must return the exact number of arguments as given in the example. The wrappers around callbacks go a long way to try to provide meaningfull error messages. If a solver fails, please check that the number of input and output arguments it takes are correct