1 files changed, 7 insertions, 3 deletions
diff --git a/src/sexp/subst.rs b/src/sexp/subst.rs
index 7cfa180..abab614 100644
--- a/src/sexp/subst.rs
+++ b/src/sexp/subst.rs
@@ -11,11 +11,15 @@ impl SExp {
     /// use melisp::sexp::{SExp::*, SLeaf::*, util::*};
     /// 
     /// assert_eq!(
-    ///     scons(Add, scons(var("a"), var("b"))).subst("b", scons(Sub, scons(2, 1))),
+    ///     scons(Add, scons(var("a"), var("b"))).subst("b", &scons(Sub, scons(2, 1))),
     ///     scons(Add, scons(var("a"), scons(Sub, scons(2, 1))))
     /// );
     /// ```
-    pub fn subst(self, name: &str, value: SExp) -> SExp {
-        todo!();
+    pub fn subst(self, name: &str, value: &SExp) -> SExp {
+        match self {
+            SCons(a, b) => scons((*a).subst(name, value), (*b).subst(name, value)),
+            Atom(Var(x)) if x == name => value.clone(),
+            t => t,
+        }
     }
 }