 reserve T for Ternary_Boolean_Algebra;
 reserve a,b,c,d,e for Element of T;
 reserve x,y,z for Element of T;
reserve T for Ternary_Boolean_Algebra;
reserve x for Element of T;

theorem TBACompl:
  for B being Boolean Lattice,
      x being Element of B,
      xx being Element of BA2TBA B st xx = x holds
   x` = xx`
  proof
    let B be Boolean Lattice,
        x be Element of B,
        xx be Element of BA2TBA B;
    assume
A1: xx = x;
    xx` = (the Compl of BA2TBA B).xx by ROBBINS1:def 3
       .= x` by A1,LATTICE4:def 13;
    hence thesis;
  end;
