Add support for reading begin/end bounds in the new standard-properties format.

src/parser/sexp.rs

@@ -74,6 +74,13 @@ enum ParseState {
 }
 
 impl<'s> Token<'s> {
+    pub fn as_vector<'p>(&'p self) -> Result<&'p Vec<Token<'s>>, Box<dyn std::error::Error>> {
+        Ok(match self {
+            Token::Vector(children) => Ok(children),
+            _ => Err(format!("wrong token type {:?}", self)),
+        }?)
+    }
+
     pub fn as_list<'p>(&'p self) -> Result<&'p Vec<Token<'s>>, Box<dyn std::error::Error>> {
         Ok(match self {
             Token::List(children) => Ok(children),