C function returning a tuple containing an array of tuples

Hello,

I have a function written in C language returning a tuple. One of the fields of
that tuple is an array of tuples.

I obtain the TupleDesc of the outer tuple using get_call_result_type(fcinfo, ...
and is working fine.

I can't figure how to obtain the TupleDesc of the tuple of the array (the 4th
attribute of the outer tuple, in my case).
For example, fcinfo->args[] array has some information about the attributes but
I haven't found anything useful to call get_call_result_type().

Any suggest?

Best regards

Alberto