 reserve L for Quasi-Boolean_Algebra,
         x, y, z for Element of L;
 reserve L for Nelson_Algebra,
         a, b, c, d, x, y, z for Element of L;

theorem Th9:
  a <= b & b <= c implies a <= c
  proof
    assume a <= b & b <= c; then
    a [= b & b [= c by LATTICES:4;
    hence thesis by LATTICES:4,7;
  end;
