Recently I read this post, which gives examples of Baire class 2 functions. I have also been reading the Wikipedia article on Baire functions. The article claims that "Henri Lebesgue proved that ... there exist functions that are not in any Baire class." I would like to see an example of such a function.
I am having a hard time finding anything that directly addresses my specific question. What I know so far is the definition: such a function is not the pointwise limit of any sequence of Baire class $\alpha$ functions, for any countable ordinal number $\alpha$. I have also found this article, which shows that any such function is not Lebesgue measurable.
I would also like to see some details of Lebesgue's proof that such functions exist. To be more specific, I would like to know whether it is constructive or is only an existence result.