Haven't seen this operator before; reminds me of angelic programming. (see eg. http://portal.acm.org/citation.cfm?id=1706339 http://portal.acm.org/citation.cfm?

Haven't seen this operator before; reminds me of angelic programming. (see eg. http://portal.acm.org/citation.cfm?id=1706339 http://portal.acm.org/citation.cfm?id=1706339) This also seems like a very natural application of monads.