reserve a,b,c for set;

theorem
  for D being non empty set,f being FinSequence of D st f=<*a,b,c*>
  holds f|1=<*a*>
proof
  let D be non empty set,f be FinSequence of D;
  assume
A1: f=<*a,b,c*>;
  f|1 = f|Seg 1 by FINSEQ_1:def 16
    .= <*a*> by A1,FINSEQ_6:4;
  hence thesis;
end;
