The SRFI restricts node indices to be non-negative integers.

1. Is there a reason that the type can't/shouldn't be implemented
allowing negative integers? SRFI 224 (integer mappings) allows negative
integers as keys, for example.

2. What is the allowed range of indices? Is an implementation allowed to
limit the indicies to the maximum fixnum (2^24, or ≈16.8 million nodes)?

-- Peter McGoron