
theorem
  for f,g being FinSequence st len f>=1 holds mid(f^g,1,len f)=f
proof
  let f,g be FinSequence;
  assume
A1: len f>=1;
  then len f-1>=0 by XREAL_1:48;
  then len f -1=len f-'1 by XREAL_0:def 2;
  then
A2: len f-'1+1=len f;
  1-1=0;
  then
A3: 1-'1=0 by XREAL_0:def 2;
  (f^g)|len f=f by FINSEQ_5:23;
  then ((f^g)/^0)|len f=f by FINSEQ_5:28;
  hence thesis by A1,A2,A3,FINSEQ_6:def 3;
end;
