theorem
  for f being FinSequence st len f is odd holds len f = 2 * Center f - 1
proof
  let f be FinSequence;
  assume len f is odd;
  then consider k being Nat such that
A1: len f = 2*k+1 by ABIAN:9;
A2: 2*k mod 2 = 0 by NAT_D:13;
  thus len f = 2 * ((2*k div 2) + (1 div 2)) + 1 by A1,Lm2,NAT_D:18
    .= 2 * (len f div 2) + (2*1 - 1) by A1,A2,NAT_D:19
    .= 2 * Center f - 1;
end;
