Constraints are a clear component of the IpOp Model presented in “Winning Opportunities, proven tools for converting your projects into success (without a business plan)” (free download on www.winning-opportunities.org). they must be respected or… circumvented.
For those who have doubt that Constraints (laws and rules) are too limiting. Here is a very good illustration of how with an agile mind they can be circumvented (effective Tactical Move):
A farmer was denied a permit to build a horse shelter. So he builds a giant table and chairs, which of course do net need permit . . .