LaTeX3: l3keys Multiple-Choices Keys and Startup Value for Predefined Choice Integer Variable

LaTeX3: l3keys Multiple-Choices Keys and Startup Value for Predefined
Choice Integer Variable

In the interface3 documentation it can be read (what follows is not a
literal quotation) that the variable \l_keys_choice_int is available to be
used as an index register for a multiple-choices key with its values
starting up from 0. Nonetheless, according to my experience with the code
below, it is as though the starting value is actually 1.
This is my class, in a file insightfully called myclass.cls:
\RequirePackage{l3keys2e,xparse}
\ProvidesExplClass
{myclass}
{2013/08/25}
{1.0}
{myclass}
\bool_new:N \g_myclass_enum_bool
\bool_new:N \g_myclass_enumalt_bool
\NewDocumentCommand \SetTBool { m }
{
\bool_gset_true:c
{ g_myclass_\tl_trim_spaces:n {#1} _bool}
}
\NewDocumentCommand \TypeBool { m }
{ \bool_if:cTF { g_myclass_#1_bool } { T } { F } }
\keys_define:nn { myclass }
{
alter .choice_code:n =
{
\AtBeginDocument{Option ~ Index: ~ \int_use:N \l_keys_choice_int\par}
\int_compare:nNnTF { \l_keys_choice_int } = { 1 }
{ \SetTBool { enum } }
{ \SetTBool { enumalt } }
},
alter .generate_choices:n = { enum, enumalt }
}
\ProcessKeysOptions { myclass }
\LoadClass{memoir}
Then I have a document file by the original name mydoc.tex reading thus:
\documentclass[alter = enum]{myclass}
\begin{document}
\TypeBool{enum}\par
\TypeBool{enumalt}
\end{document}
What I get in my typeset document is 1 T F, whereas according to the
documentation I should get 0 T F. Moreover, if I set the class option
alter = enumalt, I get 2 F T. Am I getting something wrong?