 reserve L for Lattice;
 reserve I,P for non empty ClosedSubset of L;
reserve L for lower-bounded pseudocomplemented Lattice;

theorem Th13:
  for L being lower-bounded pseudocomplemented Lattice, x being Element of L
  holds x in Skeleton L iff x** = x
  proof
    let L be lower-bounded pseudocomplemented Lattice, x be Element of L;
    hereby
      assume x in Skeleton L; then
      consider a being Element of L such that
a1:   x = a*;
      thus x** = x by a1,Th7;
    end;
    assume x** = x;
    hence x in Skeleton L;
  end;
