implemented subtraction for built-in functios

ablescript/src/variables.rs

@@ -635,7 +635,27 @@ impl ops::Sub for Value {
                     )
                     .into_functio(),
                 },
-                Functio::Builtin(_) => todo!(),
+                Functio::Builtin(b) => {
+                    let arity = b.arity;
+                    let resulting_arity = arity.saturating_sub(rhs.into_i32() as usize);
+
+                    Functio::Builtin(BuiltinFunctio::new(
+                        move |args| {
+                            b.call(
+                                &args
+                                    .iter()
+                                    .cloned()
+                                    .take(resulting_arity)
+                                    .chain(std::iter::repeat_with(|| {
+                                        Rc::new(RefCell::new(Value::Nul))
+                                    }))
+                                    .take(arity)
+                                    .collect::<Vec<_>>(),
+                            )
+                        },
+                        resulting_arity,
+                    ))
+                }
                 Functio::Chain { functios, .. } => {
                     let rhs = rhs.into_functio();
                     let (a, b) = *functios;