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=<*b,c*>
proof
  let D be non empty set,f be FinSequence of D;
  assume
A1: f = <*a,b,c*>;
  then reconsider a2=a,b2=b,c2=c as Element of D by Th10;
A2: f.3=c2 by A1;
  f.1=a2 & f.2=b2 by A1;
  hence thesis by A1,A2,FINSEQ_6:47;
end;
