From "Fundamento de Esperanto" by L.L. Zamenhof:

"16. The a of the article, and final o of substantives, may be sometimes dropped euphoniae gratia, e. g. de l' mond'o for de la mond'o; Ŝiller' for Ŝiller'o; in such cases an apostrophe should be substituted for the discarded vowel."

Nothing about poetry mentioned. In fact, on one of the first pages, Zamenhof wrote "...de l' tuta mondo..." in prose.

sudanglo:
Reading the lyrics of The Twelve days of Christmas, I thought I could polish that up a bit. Here's my go (unfinished). You might have to slightly rearrange the music.

Komence de Kristnasko,
donacis koramik'
pirarbeton kun perdrik'

Je la dua tag' Kristnaske,
donacis koramik'
du kolombajn kverajn,
kaj pirarbon kun perdrik'

Je la tria tag' Kristnaske,
donacis koramik'
Tri kokinojn francajn,
paron da palumboj,
kaj pirarbon kun perdrik'

Je la kvara tag' Kristnaske,
donacis koramik'
Kvar kantobirdojn,
tri kokinojn francajn,
paron da palumboj,
kaj pirarbon kun perdrik'

Je la kvina tag' Kristnaske,
donacis koramik'
Kvin ornamajn ringojn
Kvar kantobirdojn,
tri kokinojn francajn,
paron da palumboj,
kaj pirarbon kun perdrik'

Je la sesa tag' Kristnaske,
donacis koramik'
ses anserojn kovajn
Kvin ornamajn ringojn
Kvar kantobirdojn,
tri kokinojn francajn,
paron da palumboj,
kaj pirarbon kun perdrik'

Je la sepa tag' Kristnaske
donacis koramik'
sep cignojn naĝajn
ses anserojn kovajn
Kvin ornamajn ringojn
Kvar kantobirdojn,
tri kokinojn francajn,
paron da palumboj,
kaj pirarbon kun perdrik'

With reference to my score at "...La Dek Du Tagoj...", which is being discussed here:

Translating poetry into Esperanto is hard enough, but translating a song doubles the challenge. I wish to get the right rhyme, rhythm, meter, and meaning all at once: akin to a grand slam in baseball, or a quad spin in figure skating. I have to count the syllables of the original language, and match the Esperanto syllables note for note, accent for accent, beat for beat, rhyme for rhyme.

Thus, for example, re "Five golden rings": the English accenting is: / / - /. But if I were to sing in Esperanto "Kvin ornamajn ringojn", not only do I have two syllables too many, but the accent is out of whack: / - / - / -.

This shows how one of the problems of translating lyrics into Experanto, is getting an accent on the last syllable, when the original has it such: given that E-o words of two syllables or more are stressed on the next to last syllable. So I combine "gold" and "rings" into one word, and put "kvin" at the end, because being one syllable, that gives the right beat: "OR-RINGojn, KVIN", as with "FIVE GOLDen RINGS".

The other numbered gifts presented similar challenges in matching the beat and the syllables; and the whole project took far longer to translate and arrange than I expected...

Merry Christmas!