reserve i,j,k,l,m,n for Nat,
  D for non empty set,
  f for FinSequence of D;

theorem Th5:
  i in dom f & j in dom f implies len mid(f,i,j) >= 1
proof
A1: i <= j or j < i;
  assume i in dom f;
  then
A2: 1 <= i & i <= len f by FINSEQ_3:25;
  assume j in dom f;
  then 1 <= j & j <= len f by FINSEQ_3:25;
  then len mid(f,i,j)=i-'j+1 or len mid(f,i,j)=j-'i+1 by A2,A1,FINSEQ_6:118;
  hence thesis by NAT_1:11;
end;
