Definite Clause Grammars (DCGs) are ideally suited for reasoning about text, including parsing and also describing it.

For example, in this case, we can generate the lyrics from a declarative description:

:- use_module(library(dcgs)).
:- use_module(library(format)).

day_gift("first", "A partridge in a pear tree.").
day_gift("second", "Two turtle doves").
day_gift("third", "Three french hens").
day_gift("fourth", "Four calling birds").
day_gift("fifth", "Five golden rings").
day_gift("sixth", "Six geese a-laying").
day_gift("seventh", "Seven swans a-swimming").
day_gift("eight", "Eight maids a-milking").
day_gift("ninth", "Nine ladies dancing").
day_gift("tenth", "Ten lords a-leaping").
day_gift("eleventh", "Eleven pipers piping").
day_gift("twelth", "Twelve drummers drumming").


lyrics -->
        { findall(D-G, day_gift(D, G), DGs) },
        stanzas(DGs, []).

stanzas([], _) --> [].
stanzas([Day-Gift|DGs], Prev) -->
        format_("On the ~s day of Christmas~n", [Day]),
        "My true love gave to me:\n",
        format_("~s", [Gift]),
        previous_gifts(Prev),
        stanzas(DGs, [Gift|Prev]).

previous_gifts([]) --> "\n\n".
previous_gifts([G|Gs]) --> previous_(Gs, G).

previous_([], G) -->
        format_(" and~n~s~n~n", [G]).
previous_([G|Gs], Current) -->
        format_("~n~s", [Current]),
        previous_(Gs, G).

Sample query:

?- phrase(lyrics, Ls), format("~s", [Ls]).
On the first day of Christmas
My true love gave to me:
...

Including the binding Ls = "On the first day of ...", which can be used for reasoning about the text.

Thank you for this interesting puzzle, and Merry Christmas!