Testing mod-expt

We can use mod-expt to verify a signature:
     > (= (mod-expt signature 3 modulus) message)
     #t
How about to sign a simple one-digit message?
     > (mod-expt 7 signing-exponent modulus)
This will result in a stack overflow.

Q: What kind of repetition does mod-expt use?

A: Linear recursion

Linear recursion must use more memory for each unfinished recursive call.

Q: Since linear iteration does not use extra memory, would it be a good approach for mod-expt?