For National Poetry Day

'One

A Poem

A Raven

Midnights so dreary, tired and weary,
Silently pondering volumes extolling all by-now obsolete lore.
During my rather long nap - the weirdest tap!
An ominous vibrating sound disturbing my chamber's antedoor.
"This", I whispered quietly, "I ignore".'

Read the whole thing at http://www.cadaeic.net/cadenza.htm

If you haven't come across this before, can you work out the constraint followed by the author?