diff --git a/src/model.rs b/src/model.rs
index d6f0ea1..f9e9106 100644
--- a/src/model.rs
+++ b/src/model.rs
@@ -2,6 +2,7 @@ mod song {
     use std::{
         collections::{BTreeMap, VecDeque},
         str::FromStr,
+        sync::OnceLock,
     };
 
     use regex::Regex;
@@ -19,7 +20,7 @@ mod song {
     }
 
     /// Sequence of verse names.
-    pub type Plan = Vec<String>;
+    pub type Plan = VecDeque<String>;
 
     pub struct Song {
         pub name: String,
@@ -29,7 +30,7 @@ mod song {
     }
 
     impl Song {
-        pub fn plan(&self, plan_name: Option<String>) -> &Vec<String> {
+        pub fn plan(&self, plan_name: Option<String>) -> &VecDeque<String> {
             plan_name
                 .map(|plan_name| {
                     self.other_plans
@@ -41,11 +42,15 @@ mod song {
     }
 
     #[derive(Debug)]
-    pub enum SongParseError {
-        EmptyString,
+    pub struct SourceRef {
+        line_number: usize,
     }
 
-    const REGEX_TWO_NEWLINES_WITH_ARBITRARY_SURROUNDING_WHITESPACE: &str = r"\s*[\n\r]\s*[\n\r]\s*";
+    #[derive(Debug)]
+    pub enum SongParseError {
+        EmptyString,
+        InvalidMetadata(SourceRef),
+    }
 
     impl FromStr for Song {
         type Err = SongParseError;
@@ -55,7 +60,8 @@ mod song {
                 return Err(SongParseError::EmptyString);
             }
 
-            let re = Regex::new(REGEX_TWO_NEWLINES_WITH_ARBITRARY_SURROUNDING_WHITESPACE).unwrap();
+            static HUNK_REGEX: OnceLock<Regex> = OnceLock::new();
+            let re = HUNK_REGEX.get_or_init(|| Regex::new(r"\s*[\n\r]\s*[\n\r]\s*").unwrap());
             let mut hunks = VecDeque::new();
             let mut last_end: usize = 0;
 
@@ -68,14 +74,25 @@ mod song {
             // process header
             let mut header_lines = hunks.pop_front().unwrap().lines();
             let name = header_lines.next().unwrap().trim().to_owned();
+            let mut other_plans = BTreeMap::new();
 
-            for line in header_lines {
+            for (line_number, line) in header_lines.enumerate() {
+                if line.starts_with("map(") {
+                    match line.find(":") {
+                        Some(i) => {}
+                        None => {
+                            return Err(SongParseError::InvalidMetadata(SourceRef { line_number }))
+                        }
+                    }
+                }
                 // map(band2): slide1, slide2
                 // band2: slide1, slide2
             }
 
             let mut verses = BTreeMap::new();
 
+            let mut default_plan = Plan::new();
+
             // process verses
             for hunk in hunks {
                 let mut verse_contents = hunk;
@@ -86,14 +103,15 @@ mod song {
                 } else {
                     format!("Generated Verse {}", verses.len() + 1).to_owned()
                 };
-                verses.insert(verse_name, Verse::new(verse_contents.to_owned()));
+                verses.insert(verse_name.clone(), Verse::new(verse_contents.to_owned()));
+                default_plan.push_back(verse_name.clone());
             }
 
             Ok(Self {
                 name,
                 verses,
-                other_plans: BTreeMap::new(),
-                default_plan: vec![],
+                other_plans,
+                default_plan,
             })
         }
     }
@@ -117,8 +135,8 @@ mod song {
                 })
             );
             assert_eq!(song.verses.len(), 1);
-            assert_eq!(song.default_plan.len(), 1);
             assert_eq!(song.default_plan[0], "Generated Verse 1");
+            assert_eq!(song.default_plan.len(), 1);
         }
     }
 }