This function applies the rule that,
if the English language word or phrase for the number k
begins with a vowel, then it should be preceded by an,
and otherwise by a.
If teenhundreds=FALSE
(the default),
the numbers 1100 and 1800 will be read as
‘one thousand one hundred’ and ‘one thousand eight hundred’,
and the indefinite article will be a.
However if teenhundreds=TRUE
, the numbers 1100 and 1800
be read as ‘eleven hundred’ and ‘eighteen hundred’
and the indefinite article will be an.