reserve A for set, x,y,z for object,
  k for Element of NAT;
reserve n for Nat,
  x for object;

theorem Th11:
  for M be FinSequence st len M = n+1 holds len Del(M,n+1) = n
proof
  let M be FinSequence;
  assume
A1: len M = n+1;
  then n+1 in Seg len M by FINSEQ_1:4;
  then n+1 in dom M by FINSEQ_1:def 3;
  hence thesis by A1,FINSEQ_3:109;
end;
