regions: interval operations are recursive and they may blow up the stack

[dkochmanski]

bands-op and isum-op are recursive and (at least) for the former it is very easy to blow up the stack. These functions are also qutie unreadable, so rewriting them (and testing) would be preferable - with more intelligible comments.

Steps to reproduce (this code blows the stack on sbcl, it was extracted from the user application code):

pillton.lisp.txt