I don't understand what this block in the BIGNUM library does:
It reports itself, and is used for putting a number or string in a ring.
Yes, it reports its input (it's the identity function), but in this context its purpose isn't about rings. (In other contexts, yes.)
The bignum library provides the entire Scheme numeric tower, not just bignums. So when using it, you may want to use things such as 3/4 and 2-7i as input to arithmetic functions. But (the only exception to our general looseness about enforcing input types) oval input slots don't let you type "/" or "i." The identity block has a rectangular input slot, so you can put arbitrary strings in it.
Oh, I never knew that was what it was for. Thanks