`empty? match` λ `xs.` *this definition left as an exercise*;
`tail match` λ `xs.` *this definition left as an exercise*;
`length match` λ `xs. let`
- `n := 0;`
- `while not (empty? xs) do`
- `n := n + 1;`
- `xs := tail xs`
- `end`
- `in n`
+ `n := 0;`
+ `while not (empty? xs) do`
+ `n := n + 1;`
+ `xs := tail xs`
+ `end`
+ `in n`
+`in length`
+
+Here ...