'Switch on/off verbose mode
Parameters
verbose : bool
switch on/off verbose mode
print_func : function
A function that computes statistics of initialized arrays.
Takes an `NDArray` and returns an `str`. Defaults to mean
absolute value str((|x|/size(x)).asscalar()).'

def set_verbosity(self, verbose=False, print_func=None):

    self._verbose = verbose
    if (print_func is None):
        def asum_stat(x):
            'returns    |x|/size(x),    async    execution.'
            return str((ndarray.norm(x) / sqrt(x.size)).asscalar())
        print_func = asum_stat
    self._print_func = print_func
    return self
