The first line designates the rhyme you start with;
The second moves you quickly to another.
The third’s a crossroads: either you must part with
Rhyme a and follow Donne, or seek a’s brother.
The choice is made. Voila. You’ve gone Shakespearean,
But now, at once, a new dilemma’s posed.
The octave’s nearly finished. A criterion
Demands swift resolution: open, or closed?
Again, your choice is made. The pattern’s set.
Two rhymes remain, and you’ve found one already!
Not only that. Now, if you should forget,
Or lose your grip, the structure holds it steady.
In fact, there’s very little left to do.
A simple couplet. Rhyme it, and you’re through.