theorem Th60:
  for L being bounded distributive well-complemented
  preOrthoLattice holds (Top L)` = Bottom L
proof
  let L be bounded distributive well-complemented preOrthoLattice;
  set x = the Element of L;
  (Top L)` = (x`` + x`)` by Th59
    .= x` "/\" x by Th33
    .= Bottom L by Th59;
  hence thesis;
end;
