reserve X,x for set;
reserve k,m,n for Element of NAT,
  p,q,r,s,r9,s9 for Element of HP-WFF,
  T1,T2 for Tree;
reserve T1,T2 for DecoratedTree;
reserve t,t1 for FinSequence;

theorem Th21:
  prop n = prop m implies n = m
proof
  assume prop n = prop m;
  then 3+n = 3+m by FINSEQ_1:76;
  hence thesis;
end;
