Highlander

Rule: There can be only One

Example: Only One -- More than one alive

Given there are 3 ninjas

And there are more than one ninja alive

When 2 ninjas meet, they will fight

Then one ninja dies (but not me)

And there is one ninja less alive

Example: Only One -- One alive

Given there is only 1 ninja alive

Then he (or she) will live forever ;-)

Rule: There can be Two (in some cases)

Example: Two -- Dead and Reborn as Phoenix

Then the story continues