make unquote only apply to well formed cons lists

i wanted to delay it somehow, but this is what i really want (i understand that it's a tradeoff, but...)

src/main.rs

@@ -926,7 +926,9 @@ impl<'src> SpecialForm<'src> for Unquote<'src> {
         match sexp {
             Sexp::Atom(_) => None,
             Sexp::List(xs) => match &xs[..] {
-                [Sexp::Atom(unquote), expr] if *unquote == *UNQUOTE => Some(Self(expr)),
+                [Sexp::Atom(unquote), expr] if *unquote == *UNQUOTE && is_all_conses(expr) => {
+                    Some(Self(expr))
+                }
                 _ => None,
             },
         }
@@ -939,7 +941,22 @@ impl<'src> SpecialForm<'src> for Unquote<'src> {
     }
 }
 
-fn deconsify(sexp: &Sexp) -> Sexp {
+#[must_use]
+pub fn is_all_conses(expr: &Sexp) -> bool {
+    match expr {
+        Sexp::Atom(_) => true,
+        Sexp::List(xs) => match &xs[..] {
+            [Sexp::Atom(cons), car, cdr] if *cons == *CONS => {
+                is_all_conses(car) && is_all_conses(cdr)
+            }
+            [] => true,
+            _ => false,
+        },
+    }
+}
+
+#[must_use]
+pub fn deconsify(sexp: &Sexp) -> Sexp {
     match sexp {
         Sexp::Atom(at) => Sexp::Atom(*at),
         Sexp::List(xs) => match &xs[..] {