The first sub-pattern in an OR group is always true.
or The first sub-pattern in an AND group is always false.
In the sub-pattern, (WHITE-SPACE* OR a), sub-pattern
a will never be consulted, since the first part always is
matched, even if only zero times.
In the sub-pattern, ((LOOKAHEAD NOT WHITE-SPACE*) b, the
lookahead-not part always fails, so OmniMark never attempts to
match sub-pattern b.