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

theorem Th13:
  for a, b being Element of B_6, x, y being Element of Benzene st
  a = x & b = y holds a <= b iff x [= y
proof
  let a, b be Element of B_6, x, y be Element of Benzene;
  assume
A1: a = x & b = y;
  hereby
    assume a <= b;
    then x% <= y% by A1,Th1,Th12;
    hence x [= y by LATTICE3:7;
  end;
  assume x [= y;
  then x% <= y% by LATTICE3:7;
  hence thesis by A1,Th1,Th12;
end;
