Why does asciidoctor render these lines verbatim?

I had a problem with asciidoc and asciidoctor 
and the test file pasted in below
The asciidoc part of the problem was kindly answered for me at
https://groups.google.com/forum/#!topic/asciidoc/L7Rj0iUC3ro

That leaves the asciidoctor half of the problem:
In asciidoctor the lines
        [[2015y12m31dT23H59M59S]]
        == 1. This html; is made from infile
are rendered verbatim, as
        <div class="paragraph">
        <p>[[2015y12m31dT23H59M59S]]
        ==   1.  This html; is made from infile</p>
        </div>  
That is not at all what I expected

(In asciidoc the same lines  are rendered as a header, with anchor id,
        <div class="sect1">
        <h2 id="2015y12m31dT23H59M59S">1.  This html; is made from infile</h2>
which is what I expected)

What am I doing wrong in asciidoctor?

RichardH
TEST FILE 
= wot i did 2015
just little me

:linkattrs:

[[2015y12m31dT23H59M59S]]
==   1.  This html; is made from infile
[subs="macros"]
------------------------------------------------------------
THIS HTML; IS MADE FROM INFILE
     /home/rch/Public/Data/Calendars/wotidid/WOTIDID_2015.TXT
 by Perl script
     /home/rch/Public/Data/Calendars/wotidid/iterative_cross_referencer_V006.pl
 Nota bene; Line width should not exceed 100 characters
             For more, link:./V006_RHFrame_LONG.html#2015y12m31dT23H59M59S[click here, window="lower"]


------------------------------------------------------------

Anchor IDs in Asciidoctor must start with a character (from a to z or A to Z) and not a number.

EDIT: I checked the source to be sure, and it can also begin with '_' or ':', but not with numbers (see https://github.com/asciidoctor/asciidoctor/blob/master/lib/asciidoctor.rb#L534)

If you just write any letter or _ or : at the beginning, it works.

Thank you!
RichardH