'Return the first derivative of this function.'

def fdiff(self, argindex=1):

    x = self.args[0]
    if (len(self.args) == 1):
        if (argindex == 1):
            return (LambertW(x) / (x * (1 + LambertW(x))))
    else:
        k = self.args[1]
        if (argindex == 1):
            return (LambertW(x, k) / (x * (1 + LambertW(x, k))))
    raise ArgumentIndexError(self, argindex)
