I’d spent hours on some coding homework, so I decided to take a little break.  When I came back and tried to run the code, the computer stalled, and the “busy” signal flashed for what seems like an eternity.  Then a bunch of disorganized numerical values were returned that I had no code for.  It must have been S lurking about.  They completely disrupted my results!  I had to go back and add this line of code:

function  =  lights_on(S)

after that, my code worked and the random numbers were gone!