reserve L for Ortholattice,
  a, b, c for Element of L;

theorem Th6:
  for L being orthomodular meet-absorbing join-absorbing
join-associative meet-commutative non empty OrthoLattStr, x, y being Element
  of L holds x "\/" (x` "/\" (x "\/" y)) = x "\/" y
by LATTICES:5,Def1;
