feat: add list function and lemmas

Batteries/Data/List/Lemmas.lean

@@ -19,6 +19,15 @@ namespace List
 
 @[deprecated (since := "2024-08-15")] alias isEmpty_iff_eq_nil := isEmpty_iff
 
+/-! ### head and tail -/
+
+theorem head_cons_tail : ∀ (l : List α) (hne : l ≠ []), l.head hne :: l.tail = l
+  | _::_, _ => rfl
+
+theorem singleton_head_eq_self (l : List α) (hne : l ≠ []) (htl : l.tail = []) :
+    [l.head hne] = l := by
+  conv => rhs; rw [← head_cons_tail l hne, htl]
+
 /-! ### next? -/
 
 @[simp] theorem next?_nil : @next? α [] = none := rfl
@@ -482,6 +491,15 @@ theorem Sublist.erase_diff_erase_sublist {a : α} :
 
 end Diff
 
+/-! ### prefix, suffix, infix -/
+
+theorem singleton_prefix_cons (a) : [a] <+: a :: l :=
+  (prefix_cons_inj a).mpr nil_prefix
+
+theorem ne_nil_of_not_prefix (h : ¬l₁ <+: l₂) : l₁ ≠ [] := by
+  intro heq
+  simp [heq, nil_prefix] at h
+
 /-! ### drop -/
 
 theorem disjoint_take_drop : ∀ {l : List α}, l.Nodup → m ≤ n → Disjoint (l.take m) (l.drop n)