diff --git a/src/cities.rs b/src/cities.rs
index ff3b48f..bbc0cf9 100644
--- a/src/cities.rs
+++ b/src/cities.rs
@@ -75,6 +75,14 @@ impl Cities {
         self.keyed.len()
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.keyed.is_empty()
+    }
+
+    pub fn iter(&self) -> impl Iterator<Item = &City> {
+        self.keyed.iter().map(|k| &k.city)
+    }
+
     pub fn get(&self, id: &str) -> Option<&City> {
         self.by_id.get(id).map(|&i| &self.keyed[i].city)
     }
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..f4aa688
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,19 @@
+pub mod cities;
+pub mod config;
+pub mod db;
+pub mod dms;
+pub mod labeler;
+pub mod labels;
+pub mod server;
+
+use anyhow::{Context, Result};
+use jacquard::client::{Agent, MemoryCredentialSession};
+use jacquard::types::string::Did;
+
+/// Log in as the labeler account with an app password.
+pub async fn login(handle: &str, password: &str) -> Result<(Agent<MemoryCredentialSession>, Did)> {
+    let (session, account) = MemoryCredentialSession::authenticated(handle, password, None, None)
+        .await
+        .with_context(|| format!("logging in as {handle}"))?;
+    Ok((Agent::from(session), account.did))
+}
diff --git a/src/main.rs b/src/main.rs
index 3ffd5c1..8696231 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,38 +1,25 @@
-mod cities;
-mod config;
-mod db;
-mod dms;
-mod labeler;
-mod labels;
-mod server;
-
 use std::sync::Arc;
 
 use anyhow::{Context, Result};
-use jacquard::client::{Agent, MemoryCredentialSession};
+use cities_labeler::cities::Cities;
+use cities_labeler::config::Config;
+use cities_labeler::db::Db;
+use cities_labeler::dms::Dms;
+use cities_labeler::labeler::Labeler;
+use cities_labeler::{login, server};
 use k256::ecdsa::SigningKey;
 
-use crate::cities::Cities;
-use crate::config::Config;
-use crate::db::Db;
-use crate::dms::Dms;
-use crate::labeler::Labeler;
-
 #[tokio::main]
 async fn main() -> Result<()> {
     let config = Config::from_env()?;
 
-    let (session, account) =
-        MemoryCredentialSession::authenticated(&config.handle, &config.app_password, None, None)
-            .await
-            .with_context(|| format!("logging in as {}", config.handle))?;
-    let agent = Agent::from(session);
+    let (agent, did) = login(&config.handle, &config.app_password).await?;
 
     let key = SigningKey::from_slice(&hex::decode(&config.signing_key_hex)?)
         .context("SIGNING_KEY_HEX")?;
     let cities = Cities::load(&config.cities_path)?;
     let db = Arc::new(Db::open(&config.db_path)?);
-    let labeler = Arc::new(Labeler::new(db, account.did, key));
+    let labeler = Arc::new(Labeler::new(db, did, key));
     println!(
         "labeling as {}, loaded {} cities",
         labeler.did,