#eval "abc"

/- some "a" -/
#eval
 let s₁  := "abcde" in
 let it₁ := s₁.mk_iterator in
 let it₂ := it₁.next in
 it₁.extract it₂

/- some "" -/
#eval
 let s₁  := "abcde" in
 let it₁ := s₁.mk_iterator in
 it₁.extract it₁

/- none -/
#eval
 let s₁  := "abcde" in
 let it₁ := s₁.mk_iterator in
 let it₂ := it₁.next in
 it₂.extract it₁

/- some "abc" -/
#eval
 let s₁  := "abcde" in
 let it₁ := s₁.mk_iterator in
 let it₂ := it₁.next.next.next.prev.next in
 it₁.extract it₂

/- some "bcde" -/
#eval
 let s₁  := "abcde" in
 let it₁ := s₁.mk_iterator.next in
 let it₂ := it₁.next.next.next.next in
 it₁.extract it₂

/- some "abcde" -/
#eval
 let s₁  := "abcde" in
 let it₁ := s₁.mk_iterator in
 let it₂ := it₁.next.next.next.next.next in
 it₁.extract it₂

/- some "ab" -/
#eval
  let s₁  := "abcde" in
  let s₂  := "abcde" in
  let it₁ := s₁.mk_iterator in
  let it₂ := s₂.mk_iterator.next.next in
  it₁.extract it₂

/- none -/
#eval
  let s₁  := "abcde" in
  let s₂  := "abhde" in
  let it₁ := s₁.mk_iterator in
  let it₂ := s₂.mk_iterator.next.next in
  it₁.extract it₂

/- none -/
#eval
  let s₁  := "abcde" in
  let it₁ := s₁.mk_iterator in
  let it₂ := it₁.next.set_curr 'a' in
  it₁.extract it₂

/- some "a" -/
#eval
  let s₁  := "abcde" in
  let it₁ := s₁.mk_iterator in
  let it₂ := it₁.next.set_curr 'b' in
  it₁.extract it₂

/- some "a" -/
#eval
  let s₁  := "abcde" in
  let it₁ := s₁.mk_iterator in
  let it₂ := (it₁.next.set_curr 'a').set_curr 'b' in
  it₁.extract it₂