mturner,
Empty rectangle goes back a ways, and the oldest mentions I know of are from the Eureka forum which no longer exists. I wasn't in the Sudoku community back then. I'm basing my ER mention on my memory of posts I read on the Eureka forum before it went away. So I'm not sure I can give you as much detail as would be preferred. There may well be someone else around who participated in the original discussions who remembers it more accurately/differently than I do.
The earliest mention I can find at the moment is from Jan 2006,
here, where ER is named in a list of techniques that can form one of the kinds of nodes that can occur in Grouped Nice Loops. A relatively complete definition occurs in Feb 2006
here, in the context of Turbot Fish, where chains doesn't really enter into it. But both of these come after mentions from 2005 on the Eureka forum which no longer exist.
A good overview of ER occurs
here. It is from slightly later, and the links don't work, but it mentions ERs use both stand alone and as part of Grouped Coloring. Another interesting post is a topic from 2008 where grouped coloring and ER again come up,
here.
Another thing to keep in mind is that chains have been around for a very long time, but originally there were lots of different kinds of chains which maintained their separate identities for some time. The modern view is that a chain is a chain, and it's links can be made in a variety of ways and subsume a variety of techniques. See for example
here, where a w-wing is used as a link in a chain. There is often debate about the correct notation for these chains, but just about everyone appears to accept the validity of the chains themselves.
I don't claim that Myth Jellie's 2006 post made your technique clear. That post is the first example I can find of something with the same essential character, even if it is lacking in full generality. What is interesting is that we can get fairly close to your technique back in 2006, even if it wasn't really generalized back then.
A lot has happened since then, for example
ALS Chains (also discussed
here), and then there is
this example which pretty much nails it, as does
this one, both from much more recently.
Even so, there is still room for confusion/debate, as my references use terms like AIC, Nice Loops, etc, while you talk about medusa coloring (with extensions). I've certainly never seen ALS used with medusa coloring specifically, rather I have seen ALS used with various kinds of chains. Now personally I take medusa coloring as being a cool notational device for locating chains, but as you have seen in this topic it is very easy to become mired in questions of terminology. Neither of us are using the term medusa coloring in the traditional way and how medusa coloring corresponds to chains rather depends on which extensions you accept and which you don't and even then it is still a completely different way of looking at the situation which some would distinguish as a distinct technique regardless of the parallels.
There is also the possibility that I haven't fully grasped your description of your technique, and simply assumed it is describing a technique I already know, filling in the blanks/contradictions as needed. I will leave it to you to read some of these more recent references and tell me if any of them capture the idea.
by David P Bird » Sun Mar 17, 2013 1:32 pm 
