  The union-closed sets conjecture (Frankl's conjecture) says that for any
finite union-closed family of finite sets, other than the family consisting
only of the empty set, there exists an element that belongs to at least half of
the sets in the family. In [3], a stronger version of Frankl's conjecture
(S-Frankl conjecture for short) was introduced and a partial proof was given.
In particular, it was proved in \cite{CH17} that S-Frankl conjecture holds when
$n\leq 5$, where $n$ is the number of all the elements in the family of sets.
Now, we want to prove that it holds when $n=6$. Since the paper is very long,
we split it into two parts. This is the first part.
