diff --git a/lib/spanned_sexp/src/error.rs b/lib/spanned_sexp/src/error.rs
index f4cdca5..f3d87ad 100644
--- a/lib/spanned_sexp/src/error.rs
+++ b/lib/spanned_sexp/src/error.rs
@@ -4,6 +4,13 @@ use std::{cmp, fmt};
 pub struct Error {
     /// The error message.
     pub message: &'static str,
+    /// Position in the source string where the error was detected
+    pub pos: SourcePosition,
+}
+
+/// Position in the input string
+#[derive(Debug, PartialEq, Clone)]
+pub struct SourcePosition {
     /// The line number on which the error occurred.
     pub line: usize,
     /// The column number on which the error occurred.
@@ -25,7 +32,7 @@ pub(crate) type ERes<T> = Result<T, Err>;
 
 impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
-        write!(f, "{}:{}: {}", self.line, self.column, self.message)
+        write!(f, "{}:{}: {}", self.pos.line, self.pos.column, self.message)
     }
 }
 
@@ -37,7 +44,7 @@ impl fmt::Debug for Error {
 
 impl std::error::Error for Error {}
 
-pub(crate) fn get_line_and_column(s: &str, pos: usize) -> (usize, usize) {
+pub(crate) fn get_line_and_column(s: &str, pos: usize) -> SourcePosition {
     let mut line: usize = 1;
     let mut col: isize = -1;
     for c in s.chars().take(pos + 1) {
@@ -48,17 +55,18 @@ pub(crate) fn get_line_and_column(s: &str, pos: usize) -> (usize, usize) {
             col += 1;
         }
     }
-    (line, cmp::max(col, 0) as usize)
+    SourcePosition {
+        line,
+        column: cmp::max(col, 0) as usize,
+        index: pos,
+    }
 }
 
 #[cold]
 fn err_impl(message: &'static str, s: &str, pos: &usize) -> Err {
-    let (line, column) = get_line_and_column(s, *pos);
     Box::new(Error {
-        message: message,
-        line: line,
-        column: column,
-        index: *pos,
+        message,
+        pos: get_line_and_column(s, *pos)
     })
 }
 
@@ -66,3 +74,12 @@ fn err_impl(message: &'static str, s: &str, pos: &usize) -> Err {
 pub(crate) fn err<T>(message: &'static str, s: &str, pos: &usize) -> ERes<T> {
     Err(err_impl(message, s, pos))
 }
+
+/// Build a span
+pub(crate) fn spos(s: &str, pos: &usize) -> Option<Box<SourcePosition>> {
+    if *pos >= s.len() {
+        None
+    } else {
+        Some(Box::new(get_line_and_column(s, *pos)))
+    }
+}
diff --git a/lib/spanned_sexp/src/lib.rs b/lib/spanned_sexp/src/lib.rs
index 35dee98..d3b8c91 100644
--- a/lib/spanned_sexp/src/lib.rs
+++ b/lib/spanned_sexp/src/lib.rs
@@ -13,7 +13,8 @@ use std::fmt;
 use std::str::{self, FromStr};
 
 pub use error::Error;
-use error::{ERes, err};
+pub use error::SourcePosition;
+use error::{ERes, err, spos};
 
 #[cfg(test)]
 mod test;
@@ -34,11 +35,26 @@ pub enum Atom {
 
 /// An s-expression is either an atom or a list of s-expressions. This is
 /// similar to the data format used by lisp.
-#[derive(PartialEq, Clone, PartialOrd)]
-#[allow(missing_docs)]
+#[derive(Clone)]
 pub enum Sexp {
-    Atom(Atom),
-    List(Vec<Sexp>),
+    /// Atom
+    Atom(Atom, Option<Box<SourcePosition>>),
+    /// List of expressions
+    List(Vec<Sexp>, Option<Box<SourcePosition>>),
+}
+
+impl PartialEq for Sexp {
+    fn eq(&self, other: &Self) -> bool {
+        match (self, other) {
+            (Sexp::Atom(a, _), Sexp::Atom(b, _)) => {
+                a == b
+            }
+            (Sexp::List(a, _), Sexp::List(b, _)) => {
+                a == b
+            }
+            _ => false
+        }
+    }
 }
 
 
@@ -180,30 +196,33 @@ fn parse_sexp(s: &str, pos: &mut usize) -> ERes<Sexp> {
     trace!("parse_sexp {}", pos);
     zspace(s, pos)?;
     let (c, _) = peek(s, pos)?;
-    let r =
-        if c == '(' { Ok(Sexp::List(parse_list(s, pos)?)) } else { Ok(Sexp::Atom(parse_atom(s, pos)?)) };
+    let r = if c == '(' {
+        Ok(Sexp::List(parse_list(s, pos)?, spos(s, pos)))
+    } else {
+        Ok(Sexp::Atom(parse_atom(s, pos)?, spos(s, pos)))
+    };
     zspace(s, pos)?;
     r
 }
 
 /// Constructs an atomic s-expression from a string.
 pub fn atom_s(s: &str) -> Sexp {
-    Sexp::Atom(Atom::S(s.to_owned()))
+    Sexp::Atom(Atom::S(s.to_owned()), None)
 }
 
 /// Constructs an atomic s-expression from an int.
 pub fn atom_i(i: i64) -> Sexp {
-    Sexp::Atom(Atom::I(i))
+    Sexp::Atom(Atom::I(i), None)
 }
 
 /// Constructs an atomic s-expression from a float.
 pub fn atom_f(f: f64) -> Sexp {
-    Sexp::Atom(Atom::F(f))
+    Sexp::Atom(Atom::F(f), None)
 }
 
 /// Constructs a list s-expression given a slice of s-expressions.
 pub fn list(xs: &[Sexp]) -> Sexp {
-    Sexp::List(xs.to_owned())
+    Sexp::List(xs.to_owned(), None)
 }
 
 /// Reads an s-expression out of a `&str`.
@@ -256,8 +275,8 @@ impl fmt::Display for Atom {
 impl fmt::Display for Sexp {
     fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
         match *self {
-            Sexp::Atom(ref a) => write!(f, "{}", a),
-            Sexp::List(ref xs) => {
+            Sexp::Atom(ref a, _) => write!(f, "{}", a),
+            Sexp::List(ref xs, _) => {
                 write!(f, "(")?;
                 for (i, x) in xs.iter().enumerate() {
                     let s = if i == 0 { "" } else { " " };
diff --git a/lib/spanned_sexp/src/test.rs b/lib/spanned_sexp/src/test.rs
index ec6c23c..f96beca 100644
--- a/lib/spanned_sexp/src/test.rs
+++ b/lib/spanned_sexp/src/test.rs
@@ -27,8 +27,8 @@ fn test_pp() {
 fn test_tight_parens() {
     let s = "(hello(world))";
     let sexp = parse(s).unwrap();
-    assert_eq!(sexp, Sexp::List(vec![Sexp::Atom(Atom::S("hello".into())),
-                                     Sexp::List(vec![Sexp::Atom(Atom::S("world".into()))])]));
+    assert_eq!(sexp, Sexp::List(vec![Sexp::Atom(Atom::S("hello".into()), None),
+                                     Sexp::List(vec![Sexp::Atom(Atom::S("world".into()), None)], None)], None));
     let s = "(this (has)tight(parens))";
     let s2 = "( this ( has ) tight ( parens ) )";
     assert_eq!(parse(s).unwrap(), parse(s2).unwrap());
@@ -50,16 +50,16 @@ fn show_an_error() {
 #[test]
 fn line_and_col_test() {
     let s = "0123456789\n0123456789\n\n6";
-    assert_eq!(get_line_and_column(s, 4), (1, 4));
+    assert_eq!(get_line_and_column(s, 4), SourcePosition { line: 1, column: 4, index: 4 });
 
-    assert_eq!(get_line_and_column(s, 10), (2, 0));
-    assert_eq!(get_line_and_column(s, 11), (2, 0));
-    assert_eq!(get_line_and_column(s, 15), (2, 4));
+    assert_eq!(get_line_and_column(s, 10), SourcePosition { line: 2, column: 0, index: 10 });
+    assert_eq!(get_line_and_column(s, 11), SourcePosition { line: 2, column: 0, index: 11 });
+    assert_eq!(get_line_and_column(s, 15), SourcePosition { line: 2, column: 4, index: 15 });
 
-    assert_eq!(get_line_and_column(s, 21), (3, 0));
-    assert_eq!(get_line_and_column(s, 22), (4, 0));
-    assert_eq!(get_line_and_column(s, 23), (4, 0));
-    assert_eq!(get_line_and_column(s, 500), (4, 0));
+    assert_eq!(get_line_and_column(s, 21), SourcePosition { line: 3, column: 0, index: 21 });
+    assert_eq!(get_line_and_column(s, 22), SourcePosition { line: 4, column: 0, index: 22 });
+    assert_eq!(get_line_and_column(s, 23), SourcePosition { line: 4, column: 0, index: 23 });
+    assert_eq!(get_line_and_column(s, 500), SourcePosition { line: 4, column: 0, index: 500 });
 }
 
 #[test]