Question: Calling Procedure from Procedure

Pls see below worksheet. I want to compute the maximum number that appears in the Collatz series when starting with several starting numbers.

I calculate the maximum number appearing when starting with a specific number in procedure collatz. Then I want to calculate the maximum number appearing when calling different starting numbers in procedure collatz_max by calling collatz several times - collatz_max does not yet contain the handing of the results from collatz as the below error appears. If I only call collatz in collatz_max it return the right value, but in the loop or if I assign collatz to a variable within procedure collatz_max, calling collatz returns () as a result.

What do I do wrong?

Thanks for helping,

Oliver

Collatz.mws