Hi,

Finally, I don't have time to dig into org-element.el for the time
being.

Nonetheless, please find attached a revised version of the patch with
your comments taken into account.

In order to avoid creating the regexp each time `org-at-clock-log-p' is
called, I added the constant `org-clock-line-re', following the same
naming convention as `org-planning-or-clock-line-re' defined above.