This:

• 5 printed "abcd" and returned to 4;
• 4's loop over (d) was exhausted and returned to 3;
• 3 looping over (c,d) stepped to the second element and called 4, i.e. perm("abd", "c")
• 4 loops over (c) and calls 5;
• 5 printed "abdc" and returned to 4;

This is what triggered understanding in my brain.

Thank you!

Your explanation was just what I needed! I really appreciate the effort you went through to help me get this. Thanks.