Question: How does PDEtools/NumerDenom works?

I am doing some lengthy computation using the externally developed (and published in academic journals) package 'Janet' to compute the compatibility conditions of an overdetermined set of linear PDEs.

At some point in the process (for a set of equations rather, but not SO, complicated) the procedure gives the error

in (PDEtools/NumerDenom) too many level of recursion

I have contacted the developer of the packages and he told me that he never directly calls PDEtools functions, so he deems that that call is made by some more standard maple command (such as simplify).

I could not find any documentation of NumerDenom function, to try and understand what might be the problem (my set of equations has some extremely 'bad' fractions, but I hope this is not that naive)

How does NumerDenom works, and where is it called by 'standard' maple command?

Thank you all.