diff --git a/src/lexer.rs b/src/lexer.rs
index 984c62d..24be067 100644
--- a/src/lexer.rs
+++ b/src/lexer.rs
@@ -3,6 +3,8 @@
 use std::fmt;
 
 use characters;
+use characters::CharAt;
+use characters::Lexable;
 use characters::RelativeIndexable;
 
 pub enum Kind {
@@ -27,6 +29,15 @@ pub struct Token {
     value: String,
 }
 
+impl Token {
+    fn new(kind: Kind, value: String) -> Token {
+        Token {
+            kind: kind,
+            value: value,
+        }
+    }
+}
+
 impl fmt::Display for Token {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f, "({}, \"{}\")", self.kind, self.value)
@@ -64,26 +75,50 @@ impl Lexer {
 
     /// Advance the forward pointer to the next character.
     fn advance(&mut self) {
-        if let Some(next) = self.input.index_after(&self.forward) {
-            self.forward = next;
-        }
+        self.forward = self.input.index_after(self.forward);
+        println!("> forward={}", self.forward);
     }
 
     /// Retract the forward pointer to the previous character.
     fn retract(&mut self) {
-        if let Some(prev) = self.input.index_before(&self.forward) {
-            self.forward = prev;
-        }
+        self.forward = self.input.index_before(self.forward);
+        println!("< forward={}", self.forward);
+    }
+
+    fn advance_begin(&mut self) {
+        self.begin = self.input.index_after(self.forward);
+        println!("> begin={}", self.begin);
+    }
+
+    fn value(&self) -> String {
+        self.input[self.begin .. self.forward].to_string()
     }
 }
 
 impl Lexer {
-    fn state_initial(&mut self) {
-        println!("Initial!");
+    fn state_initial(&mut self, c: char, token: &mut Option<Token>) {
+        println!("Initial! c='{}'", c);
+        if c.is_left_paren() {
+            *token = Some(Token::new(Kind::LeftParen, c.to_string()));
+        }
+        else if c.is_right_paren() {
+            *token = Some(Token::new(Kind::RightParen, c.to_string()));
+        }
+        else if c.is_identifier_initial() {
+            self.state = State::Identifier;
+            self.advance();
+        }
     }
 
-    fn state_identifier(&mut self) {
-        println!("Identifier!")
+    fn state_identifier(&mut self, c: char, token: &mut Option<Token>) {
+        if c.is_identifier_subsequent() {
+            // State in Identifier state.
+            self.advance();
+        }
+        else {
+            *token = Some(Token::new(Kind::Identifier, self.value()));
+            self.retract();
+        }
     }
 }
 
@@ -92,16 +127,28 @@ impl Iterator for Lexer {
 
     fn next(&mut self) -> Option<Token> {
         self.begin_lexing();
-        let mut emit = false;
-        println!("Lexing '{}'", self.input);
-        while !emit {
-            match self.state {
-                State::Initial => self.state_initial(),
-                State::Identifier => self.state_identifier(),
-            }
-            emit = true;
+        if self.begin == self.input.len() {
+            return None;
         }
-        None
+        let mut token: Option<Token> = None;
+        println!("Lexing '{}'", &self.input[self.begin ..]);
+        loop {
+            if let Some(c) = self.input.char_at(self.forward) {
+                match self.state {
+                    State::Initial => self.state_initial(c, &mut token),
+                    State::Identifier => self.state_identifier(c, &mut token),
+                }
+            }
+            else {
+                assert!(false, "Invalid character! :-(");
+            }
+            if token.is_some() {
+                break;
+            }
+        }
+        self.advance_begin();
+        assert!(token.is_some());
+        token
     }
 }