reserve x,y,x1,x2,z for set,
  n,m,k for Nat,
  t1 for (DecoratedTree of [: NAT,NAT :]),
  w,s,t,u for FinSequence of NAT,
  D for non empty set;
reserve s9,w9,v9 for Element of NAT*;
reserve p,q for MP-variable;
reserve A,A1,B,B1,C,C1 for MP-wff;

theorem Th28:
  'not' A = 'not' B implies A = B
proof
  assume
A1: 'not' A = 'not' B;
  <*0*> in dom((elementary_tree 1)-->[1,0]) by Lm3;
  hence thesis by A1,Th7;
end;
