dropdown proof in jupyterbook

[MPenaR]

I'm trying to write a book where the proofs do not distract from the rest of the content. As so, I am writting every proposition and proof as:

$$
\frac{1}{a+b} = \frac{1}{a} + \frac{1}{b}
$$

:class: dropdown

its easy to check its equality for 

$$
a=b=0
$$ 

and for 

$$
a=b=\infty
$$ 

The veracity for any other value of $a$ and $b$ can be proved by interpolation. This is left to the reader.

However, when I build the book the proof is shown without the option to hide it. (That does not happen if I simply use

...

That is, in this case everything works fine. My _config.yml looks like this:

sphinx:
  extra_extensions          : # A list of extra extensions to load by Sphinx (added to those already used by JB).
    - sphinx_proof
    - sphinx_togglebutton    
  local_extensions          :   # A list of local extensions to load by sphinx specified by "name: path" items
  recursive_update          : false # A boolean indicating whether to overwrite the Sphinx config (true) or recursively update (false)
  config                    :  
    language: es # key-value pairs to directly over-ride the Sphinx configuration

I added the "sphinx_togglebutton" thinking that would fix it, but it didn't.

[MPenaR]

I'm sorry, markdown rendered the " {prf:proof}" and the "{prf:proposition}"