diff --git a/crsn/crsn-sexp/src/error.rs b/crsn/crsn-sexp/src/error.rs
index 8a3ada2..640903a 100644
--- a/crsn/crsn-sexp/src/error.rs
+++ b/crsn/crsn-sexp/src/error.rs
@@ -1,5 +1,6 @@
-use std::{cmp, fmt};
-use std::fmt::{Formatter, Debug};
+use std::{fmt};
+use super::SourcePosition;
+use position::get_line_and_column;
 
 /// The representation of an s-expression parse error.
 pub struct Error {
@@ -9,38 +10,6 @@ pub struct Error {
     pub pos: SourcePosition,
 }
 
-/// Position in the input string
-#[derive(PartialEq, Clone, Default)]
-pub struct SourcePosition {
-    /// The line number on which the error occurred.
-    pub line: u32,
-    /// The column number on which the error occurred.
-    pub column: u32,
-    /// The index in the given string which caused the error.
-    pub index: u32,
-}
-
-impl fmt::Display for SourcePosition {
-    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
-        write!(f, "{}:{}", self.line, self.column)
-    }
-}
-
-impl Debug for SourcePosition {
-    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        if f.alternate() {
-            f.debug_struct("SourcePosition")
-                .field("line", &self.line)
-                .field("column", &self.column)
-                .field("index", &self.index)
-                .finish()
-        } else {
-            // shorter version
-            write!(f, "Pos({}:{})", self.line, self.column)
-        }
-    }
-}
-
 /// Since errors are the uncommon case, they're boxed. This keeps the size of
 /// structs down, which helps performance in the common case.
 ///
@@ -66,24 +35,6 @@ impl fmt::Debug for Error {
 
 impl std::error::Error for Error {}
 
-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) {
-        if c == '\n' {
-            line += 1;
-            col = -1;
-        } else {
-            col += 1;
-        }
-    }
-    SourcePosition {
-        line: line as u32,
-        column: cmp::max(col, 0) as u32,
-        index: pos as u32,
-    }
-}
-
 #[cold]
 fn err_impl(message: &'static str, s: &str, pos: usize) -> Err {
     Box::new(Error {
@@ -96,12 +47,3 @@ 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) -> SourcePosition {
-    if pos >= s.len() {
-        Default::default()
-    } else {
-        get_line_and_column(s, pos)
-    }
-}
diff --git a/crsn/crsn-sexp/src/lib.rs b/crsn/crsn-sexp/src/lib.rs
index 50815ab..f5d6bc8 100644
--- a/crsn/crsn-sexp/src/lib.rs
+++ b/crsn/crsn-sexp/src/lib.rs
@@ -8,14 +8,16 @@ use std::borrow::Cow;
 use std::fmt;
 use std::str::{self, FromStr};
 
-use error::{ERes, err, spos};
+use error::{ERes, err};
 pub use error::Error;
-pub use error::SourcePosition;
+pub use position::SourcePosition;
+use position::spos;
 
 #[cfg(test)]
 mod test;
 
 mod error;
+mod position;
 
 /// A single data element in an s-expression. Floats are excluded to ensure
 /// atoms may be used as keys in ordered and hashed data structures.
diff --git a/crsn/crsn-sexp/src/position.rs b/crsn/crsn-sexp/src/position.rs
new file mode 100644
index 0000000..f54859c
--- /dev/null
+++ b/crsn/crsn-sexp/src/position.rs
@@ -0,0 +1,65 @@
+use std::{fmt, cmp};
+use std::fmt::{Formatter, Debug};
+
+/// Position in the input string
+#[derive(PartialEq, Clone, Default)]
+pub struct SourcePosition {
+    /// The line number on which the error occurred.
+    pub line: u32,
+    /// The column number on which the error occurred.
+    pub column: u32,
+    /// The index in the given string which caused the error.
+    pub index: u32,
+    /// File index if there are multiple files
+    pub file: u32,
+}
+
+impl fmt::Display for SourcePosition {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        write!(f, "{}:{}", self.line, self.column)
+    }
+}
+
+impl Debug for SourcePosition {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        if f.alternate() {
+            f.debug_struct("SourcePosition")
+                .field("line", &self.line)
+                .field("column", &self.column)
+                .field("index", &self.index)
+                .field("file", &self.file)
+                .finish()
+        } else {
+            // shorter version
+            write!(f, "Pos({}:{})", self.line, self.column)
+        }
+    }
+}
+
+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) {
+        if c == '\n' {
+            line += 1;
+            col = -1;
+        } else {
+            col += 1;
+        }
+    }
+    SourcePosition {
+        line: line as u32,
+        column: cmp::max(col, 0) as u32,
+        index: pos as u32,
+        file: 0
+    }
+}
+
+/// Build a span
+pub(crate) fn spos(s: &str, pos: usize) -> SourcePosition {
+    if pos >= s.len() {
+        Default::default()
+    } else {
+        get_line_and_column(s, pos)
+    }
+}
diff --git a/crsn/crsn-sexp/src/test.rs b/crsn/crsn-sexp/src/test.rs
index 3f71cb8..b7e8a86 100644
--- a/crsn/crsn-sexp/src/test.rs
+++ b/crsn/crsn-sexp/src/test.rs
@@ -1,5 +1,6 @@
 use super::*;
 use super::error::get_line_and_column;
+use super::position::get_line_and_column;
 
 #[test]
 fn test_hello_world() {
@@ -67,16 +68,16 @@ fn show_an_error() {
 #[test]
 fn line_and_col_test() {
     let s = "0123456789\n0123456789\n\n6";
-    assert_eq!(get_line_and_column(s, 4), SourcePosition { line: 1, column: 4, index: 4 });
+    assert_eq!(get_line_and_column(s, 4), SourcePosition { line: 1, column: 4, index: 4, file: 0 });
 
-    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, 10), SourcePosition { line: 2, column: 0, index: 10, file: 0 });
+    assert_eq!(get_line_and_column(s, 11), SourcePosition { line: 2, column: 0, index: 11, file: 0 });
+    assert_eq!(get_line_and_column(s, 15), SourcePosition { line: 2, column: 4, index: 15, file: 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 });
+    assert_eq!(get_line_and_column(s, 21), SourcePosition { line: 3, column: 0, index: 21, file: 0 });
+    assert_eq!(get_line_and_column(s, 22), SourcePosition { line: 4, column: 0, index: 22, file: 0 });
+    assert_eq!(get_line_and_column(s, 23), SourcePosition { line: 4, column: 0, index: 23, file: 0 });
+    assert_eq!(get_line_and_column(s, 500), SourcePosition { line: 4, column: 0, index: 500, file: 0 });
 }
 
 #[test]
diff --git a/crsn/src/runtime/program.rs b/crsn/src/runtime/program.rs
index c2c1d05..e7f8599 100644
--- a/crsn/src/runtime/program.rs
+++ b/crsn/src/runtime/program.rs
@@ -100,6 +100,7 @@ impl Program {
                     line: 0,
                     column: 0,
                     index: 0,
+                    file: 0,
                 },
                 cond: None,
             }
diff --git a/crsn/src/runtime/run_thread.rs b/crsn/src/runtime/run_thread.rs
index 89798d5..58a39a7 100644
--- a/crsn/src/runtime/run_thread.rs
+++ b/crsn/src/runtime/run_thread.rs
@@ -54,12 +54,12 @@ impl RunThread {
 
         let rs = RunState {
             thread_info: ti.clone(),
-            cr: CoroutineContext {
+            cr: Box::new(CoroutineContext {
                 handle: 0, // this is the root
                 frame: StackFrame::new(params.pc, params.args),
                 call_stack: vec![],
                 cr_state: Default::default(),
-            },
+            }),
             parked: Default::default(),
             global_regs: [0; REG_COUNT],
             ext_data: Default::default(),
diff --git a/crsn/src/runtime/run_thread/state.rs b/crsn/src/runtime/run_thread/state.rs
index 81f8185..b0d86d8 100644
--- a/crsn/src/runtime/run_thread/state.rs
+++ b/crsn/src/runtime/run_thread/state.rs
@@ -17,9 +17,9 @@ use std::time::{Instant, Duration};
 pub struct RunState {
     pub thread_info: Arc<ThreadInfo>,
     /// The active coroutine
-    pub cr: CoroutineContext,
+    pub cr: Box<CoroutineContext>,
     /// Parked coroutines
-    pub parked: VecDeque<CoroutineContext>,
+    pub parked: VecDeque<Box<CoroutineContext>>,
     /// General purpose registers that stay valid for the entire run-time of the thread
     pub global_regs: [Value; REG_COUNT],
     /// Extension data
@@ -87,12 +87,12 @@ impl RunState {
         trace!("Spawn cr {:#}", handle);
 
         // front - so it runs ASAP
-        self.parked.push_front(CoroutineContext {
+        self.parked.push_front(Box::new(CoroutineContext {
             handle,
             frame,
             call_stack: vec![],
             cr_state: Default::default()
-        });
+        }));
         if self.cr_deadline.is_none() {
             // start context switching
             self.start_task_switching();