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

theorem Th7:
  i in dom f & j in dom f implies mid(f,i,j) is non empty
proof
  assume i in dom f & j in dom f;
  then len mid(f,i,j) >= 1 by Th5;
  hence thesis by FINSEQ_3:25,RELAT_1:38;
end;
