reserve i,j,k,n for Nat;
reserve D for non empty set,
  p for Element of D,
  f,g for FinSequence of D;

theorem Th20:
  for f being FinSequence st f is non empty holds f|1 = <*f.1*>
proof
  let f be FinSequence;
  assume f is non empty;
  then 1 in dom f by Th6;
  then 1 <= len f by FINSEQ_3:25; then
A1: len(f|1) = 1 by FINSEQ_1:59;
  (f|Seg 1).1 = f.1 by FINSEQ_1:1,FUNCT_1:49;
  hence thesis by A1,FINSEQ_1:40;
end;
