diff --git a/bidir/src/bidir_debruijn.rs b/bidir/src/bidir_debruijn.rs
index 34b2aa7..c026266 100644
--- a/bidir/src/bidir_debruijn.rs
+++ b/bidir/src/bidir_debruijn.rs
@@ -1,7 +1,7 @@
-use anyhow::{bail, Result};
+use anyhow::{bail, ensure, Result};
 
 use crate::data_debruijn::{
-  Context, ContextEntry, ContextIndex, Monotype, Term, Type,
+  Context, ContextEntry, ContextIndex, Monotype, Term, Type, Visitor,
 };
 use crate::DEPTH;
 
@@ -105,6 +105,11 @@ pub fn instantiate_left(
           .get_mut(b.debruijn_level)
           .expect("should exist");
 
+        ensure!(
+          matches!(b_entry, ContextEntry::ExistentialVar),
+          "not an existential variable"
+        );
+
         *b_entry =
           ContextEntry::ExistentialSolved(Monotype::Existential(a.to_owned()));
       }
@@ -173,37 +178,59 @@ pub fn synthesize(ctx: &Context, term: &Term) -> Result<(Type, Context)> {
 
     // →I⇒' rule
     Term::Lam(e) => {
-      let (aug_ctx, marker_idx) = ctx.add(ContextEntry::Marker);
-      let (aug_ctx_at_a, ex_a_idx) = aug_ctx.add(ContextEntry::ExistentialVar);
+      let (aug_ctx, marker_idx) = ctx.add(ContextEntry::Marker)?;
+      let (aug_ctx_at_a, ex_a_idx) =
+        aug_ctx.add(ContextEntry::ExistentialVar)?;
       let (aug_ctx_at_b, ex_b_idx) =
-        aug_ctx_at_a.add(ContextEntry::ExistentialVar);
+        aug_ctx_at_a.add(ContextEntry::ExistentialVar)?;
 
       let ex_a = Type::Existential(ex_a_idx.clone());
       let ex_b = Type::Existential(ex_b_idx.clone());
 
       let (aug_ctx, _annot_idx) =
-        aug_ctx_at_b.add(ContextEntry::TermAnnot(ex_a.clone()));
+        aug_ctx_at_b.add(ContextEntry::TermAnnot(ex_a.clone()))?;
 
-      let ex_a_2 = ex_b.bump_indexes(&aug_ctx_at_a, &aug_ctx, 2);
-      let ex_b_2 = ex_b.bump_indexes(&aug_ctx_at_b, &aug_ctx, 1);
+      struct V(usize, usize);
+      impl Visitor for V {
+        fn visit_index(&self, index: &mut ContextIndex) -> Result<()> {
+          index.debruijn_index += self.0;
+          index.debruijn_level = self.1 - 1 - index.debruijn_index;
+          Ok(())
+        }
+      }
 
-      // TODO: Rewrite all indexes in this term
-      let e_2 = e.bump_indexes(ctx, &aug_ctx, 0);
+      let ex_a_2 = ex_b.transform_with_visitor(&V(2, aug_ctx.size()))?;
+      let ex_b_2 = ex_b.transform_with_visitor(&V(1, aug_ctx.size()))?;
+      let e_2 = e.transform_with_visitor(&V(0, aug_ctx.size()))?;
 
+      println!("---");
       let wtf_ctx = typecheck(&aug_ctx, &e_2, &ex_b_2)?;
 
       let (before_marker, after_marker) = wtf_ctx.split_at(&marker_idx);
-      let tau = Type::Arrow(Box::new(ex_a_2), Box::new(ex_b));
-      let mut tau = app_ctx(&after_marker, &tau)?;
 
-      let (final_ctx, gen_idx) =
-        before_marker.add(ContextEntry::ExistentialVar);
-
-      for index in after_marker.unsolved_existentials() {
-        tau = tau.subst(&index, Type::Var(gen_idx.clone()));
+      // Bump the levels down since we're splitting teh array
+      struct V2(usize);
+      impl Visitor for V2 {
+        fn visit_index(&self, index: &mut ContextIndex) -> Result<()> {
+          index.debruijn_level -= 1;
+          index.debruijn_index = self.0 - 1 - index.debruijn_level;
+          Ok(())
+        }
       }
+      let after_marker =
+        after_marker.transform_with_visitor(&V2(after_marker.size()))?;
 
-      Ok((Type::Polytype(Box::new(tau)), final_ctx))
+      let tau = Type::Arrow(Box::new(ex_a_2), Box::new(ex_b));
+      println!("pre-tau: {tau:?} (in {after_marker:?})");
+      let tau = app_ctx(&after_marker, &tau)?;
+
+      // let (final_ctx, gen_idx) =
+      //   before_marker.add(ContextEntry::ExistentialVar);
+      // for index in after_marker.unsolved_existentials() {
+      //   tau = tau.subst(&index, Type::Var(gen_idx.clone()));
+      // }
+
+      Ok((Type::Polytype(Box::new(tau)), before_marker))
     }
 
     Term::Annot(_, _) => todo!(),
@@ -224,7 +251,7 @@ pub fn app_synthesize(
 
     // ∀App rule
     (Type::Polytype(ty_a), e) => {
-      let (aug_ctx, a_idx) = ctx.add(ContextEntry::ExistentialVar);
+      let (aug_ctx, a_idx) = ctx.add(ContextEntry::ExistentialVar)?;
       let aug_ty = ty_a.subst(&a_idx, Type::Existential(a_idx.clone()));
       let (ty, ctx_delta) = app_synthesize(&aug_ctx, &aug_ty, e)?;
 
diff --git a/bidir/src/data_debruijn.rs b/bidir/src/data_debruijn.rs
index bfc9ad6..bdf9f57 100644
--- a/bidir/src/data_debruijn.rs
+++ b/bidir/src/data_debruijn.rs
@@ -1,5 +1,6 @@
 use std::fmt::{self};
 
+use anyhow::Result;
 use im::Vector;
 
 use crate::{data, gensym::gensym_type};
@@ -37,36 +38,57 @@ impl fmt::Debug for Term {
 }
 
 impl Term {
-  pub fn bump_indexes(
+  // pub fn bump_indexes(
+  //   &self,
+  //   old_ctx: &Context,
+  //   new_ctx: &Context,
+  //   by: usize,
+  // ) -> Term {
+  //   match self {
+  //     Term::Unit => Term::Unit,
+  //     Term::Var(index) => {
+  //       let debruijn_index = index.debruijn_index + by;
+  //       let debruijn_level = new_ctx.vector.len() - 1 - debruijn_index;
+  //       let new_index = ContextIndex {
+  //         debruijn_index,
+  //         debruijn_level,
+  //       };
+  //       Term::Var(new_index)
+  //     }
+  //     Term::Lam(body) => {
+  //       Term::Lam(Box::new(body.bump_indexes(old_ctx, new_ctx, by)))
+  //     }
+  //     Term::App(e1, e2) => Term::App(
+  //       Box::new(e1.bump_indexes(old_ctx, new_ctx, by)),
+  //       Box::new(e2.bump_indexes(old_ctx, new_ctx, by)),
+  //     ),
+  //     Term::Annot(e, ty) =>
+  //     // TODO: Should i bump indexes in the type too (PROBABLY)
+  //     {
+  //       Term::Annot(Box::new(e.bump_indexes(old_ctx, new_ctx, by)), ty.clone())
+  //     }
+  //   }
+  // }
+
+  pub fn transform_with_visitor<V: Visitor>(
     &self,
-    old_ctx: &Context,
-    new_ctx: &Context,
-    by: usize,
-  ) -> Term {
-    match self {
-      Term::Unit => Term::Unit,
+    visitor: &V,
+  ) -> Result<Term> {
+    let mut expr = self.clone();
+    visitor.visit_term_before(&mut expr)?;
+    let mut expr = match expr {
       Term::Var(index) => {
-        let debruijn_index = index.debruijn_index + by;
-        let debruijn_level = new_ctx.vector.len() - 1 - debruijn_index;
-        let new_index = ContextIndex {
-          debruijn_index,
-          debruijn_level,
-        };
-        Term::Var(new_index)
+        let mut index = index.clone();
+        visitor.visit_index(&mut index)?;
+        Term::Var(index)
       }
-      Term::Lam(body) => {
-        Term::Lam(Box::new(body.bump_indexes(old_ctx, new_ctx, by)))
-      }
-      Term::App(e1, e2) => Term::App(
-        Box::new(e1.bump_indexes(old_ctx, new_ctx, by)),
-        Box::new(e2.bump_indexes(old_ctx, new_ctx, by)),
-      ),
-      Term::Annot(e, ty) =>
-      // TODO: Should i bump indexes in the type too (PROBABLY)
-      {
-        Term::Annot(Box::new(e.bump_indexes(old_ctx, new_ctx, by)), ty.clone())
-      }
-    }
+      Term::Lam(_) => todo!(),
+      Term::App(_, _) => todo!(),
+      Term::Annot(_, _) => todo!(),
+      _ => expr,
+    };
+    visitor.visit_term_after(&mut expr)?;
+    Ok(expr)
   }
 }
 
@@ -116,31 +138,23 @@ impl Type {
     self.try_into_mono().is_some()
   }
 
-  pub fn bump_indexes(
-    &self,
-    old_ctx: &Context,
-    new_ctx: &Context,
-    by: usize,
-  ) -> Type {
-    match self {
-      Type::Unit => Type::Unit,
-      Type::Var(_) => todo!(),
-      Type::Existential(index) => {
-        let debruijn_index = index.debruijn_index + by;
-        let debruijn_level = new_ctx.vector.len() - 1 - debruijn_index;
-        let new_index = ContextIndex {
-          debruijn_index,
-          debruijn_level,
-        };
-        Type::Existential(new_index)
-      }
-      Type::Polytype(_) => todo!(),
-      Type::Arrow(_, _) => todo!(),
-    }
-  }
-}
+  // pub fn bump_indexes(&self, by: usize) -> Type {
+  //   match self {
+  //     Type::Unit => Type::Unit,
+  //     Type::Var(_) => todo!(),
+  //     Type::Existential(index) => {
+  //       let debruijn_index = index.debruijn_index + by;
+  //       let new_index = ContextIndex {
+  //         debruijn_index,
+  //         debruijn_level: index.debruijn_level,
+  //       };
+  //       Type::Existential(new_index)
+  //     }
+  //     Type::Polytype(_) => todo!(),
+  //     Type::Arrow(_, _) => todo!(),
+  //   }
+  // }
 
-impl Type {
   // pub fn free_vars(&self) -> HashSet<FreeVar> {
   //   fn free_vars_with_context(ctx: &Context, ty: &Type) -> HashSet<FreeVar> {
   //     match ty {
@@ -203,6 +217,27 @@ impl Type {
       ),
     }
   }
+
+  pub fn transform_with_visitor<V: Visitor>(
+    &self,
+    visitor: &V,
+  ) -> Result<Type> {
+    let mut ty = self.clone();
+    visitor.visit_type_before(&mut ty)?;
+    let mut ty = match ty {
+      Type::Var(_) => todo!(),
+      Type::Existential(index) => {
+        let mut index = index.clone();
+        visitor.visit_index(&mut index)?;
+        Type::Existential(index)
+      }
+      Type::Polytype(_) => todo!(),
+      Type::Arrow(_, _) => todo!(),
+      _ => ty,
+    };
+    visitor.visit_type_after(&mut ty)?;
+    Ok(ty)
+  }
 }
 
 #[derive(Debug, Clone)]
@@ -224,6 +259,54 @@ impl Monotype {
       }
     }
   }
+
+  // pub fn bump_indexes(&self, by: isize) -> Monotype {
+  //   match self {
+  //     Monotype::Unit => Monotype::Unit,
+  //     Monotype::Var(index) => {
+  //       let debruijn_index = (index.debruijn_index as isize + by) as usize;
+  //       // let debruijn_level = new_ctx.vector.len() - 1 - debruijn_index;
+  //       let new_index = ContextIndex {
+  //         debruijn_index,
+  //         debruijn_level: index.debruijn_level,
+  //       };
+  //       Monotype::Var(new_index)
+  //     }
+  //     Monotype::Existential(index) => {
+  //       let debruijn_index = (index.debruijn_index as isize + by) as usize;
+  //       // let debruijn_level = new_ctx.vector.len() - 1 - debruijn_index;
+  //       let new_index = ContextIndex {
+  //         debruijn_index,
+  //         debruijn_level: index.debruijn_level,
+  //       };
+  //       Monotype::Var(new_index)
+  //     }
+  //     Monotype::Arrow(a, b) => Monotype::Arrow(
+  //       Box::new(a.bump_indexes(by)),
+  //       Box::new(b.bump_indexes(by)),
+  //     ),
+  //   }
+  // }
+
+  pub fn transform_with_visitor<V: Visitor>(
+    &self,
+    visitor: &V,
+  ) -> Result<Monotype> {
+    let mut ty = self.clone();
+    visitor.visit_monotype_before(&mut ty)?;
+    let mut ty = match ty {
+      Monotype::Var(_) => todo!(),
+      Monotype::Existential(index) => {
+        let mut index = index.clone();
+        visitor.visit_index(&mut index)?;
+        Monotype::Existential(index)
+      }
+      Monotype::Arrow(_, _) => todo!(),
+      _ => ty,
+    };
+    visitor.visit_monotype_after(&mut ty)?;
+    Ok(ty)
+  }
 }
 
 #[derive(Clone)]
@@ -289,18 +372,34 @@ impl Default for Context {
 }
 
 impl Context {
-  pub fn add(&self, item: ContextEntry) -> (Context, ContextIndex) {
+  #[inline]
+  pub fn size(&self) -> usize {
+    self.vector.len()
+  }
+
+  pub fn add(&self, item: ContextEntry) -> Result<(Context, ContextIndex)> {
     let idx = self.vector.len();
-    let mut new_ctx = self.bump_indexes(1);
+    struct V;
+    impl Visitor for V {
+      fn visit_index(&self, index: &mut ContextIndex) -> Result<()> {
+        index.debruijn_index += 1;
+        Ok(())
+      }
+    }
+    let mut new_ctx = self.transform_with_visitor(&V)?;
     new_ctx.vector.push_back(item);
 
     let idx = ContextIndex {
       debruijn_index: 0,
       debruijn_level: idx,
     };
-    (new_ctx, idx)
+    Ok((new_ctx, idx))
   }
 
+  // /// Pops items from the end of the context, changing all of the indexes in the
+  // /// process
+  // pub fn pop(&self) -> Context {}
+
   pub fn get<'a>(&'a self, index: &ContextIndex) -> Option<&ContextEntry> {
     let item = self.vector.get(index.debruijn_level)?;
     Some(item)
@@ -347,10 +446,7 @@ impl Context {
 
   /// Splits the context
   pub fn split_at(&self, index: &ContextIndex) -> (Context, Context) {
-    let (before, after) = {
-      // let idx = self.vector.iter().position(p)?;
-      self.vector.clone().split_at(index.debruijn_level)
-    };
+    let (before, after) = self.vector.clone().split_at(index.debruijn_level);
 
     (Context { vector: before }, Context { vector: after })
   }
@@ -411,21 +507,56 @@ impl Context {
       .all(|item| ensure_item_validity(self, item))
   }
 
-  pub fn bump_indexes(&self, by: usize) -> Context {
-    Context {
+  pub fn transform_with_visitor<V: Visitor>(
+    &self,
+    visitor: &V,
+  ) -> Result<Context> {
+    Ok(Context {
       vector: self
         .vector
         .iter()
-        .map(|entry| match entry {
-          ContextEntry::TypeVar => ContextEntry::TypeVar,
-          ContextEntry::TermAnnot(_) => todo!(),
-          ContextEntry::ExistentialVar => ContextEntry::ExistentialVar,
-          ContextEntry::ExistentialSolved(_) => todo!(),
-          ContextEntry::Marker => ContextEntry::Marker,
+        .map(|entry| {
+          let mut entry = entry.clone();
+          visitor.visit_context_entry_before(&mut entry)?;
+          let mut entry = match entry {
+            ContextEntry::TermAnnot(ty) => {
+              ContextEntry::TermAnnot(ty.transform_with_visitor(visitor)?)
+            }
+            ContextEntry::ExistentialSolved(ty) => {
+              ContextEntry::ExistentialSolved(
+                ty.transform_with_visitor(visitor)?,
+              )
+            }
+            _ => entry,
+          };
+          visitor.visit_context_entry_after(&mut entry)?;
+          Ok(entry)
         })
-        .collect(),
-    }
+        .collect::<Result<_>>()?,
+    })
   }
+
+  // pub fn bump_indexes(&self, by: isize) -> Context {
+  //   Context {
+  //     vector: self
+  //       .vector
+  //       .iter()
+  //       .map(|entry| match entry {
+  //         ContextEntry::TypeVar => ContextEntry::TypeVar,
+  //         ContextEntry::TermAnnot(ty) => {
+  //           todo!()
+  //           // ContextEntry::TermAnnot(ty.bump_indexes(by))
+  //         }
+  //         ContextEntry::ExistentialVar => ContextEntry::ExistentialVar,
+  //         ContextEntry::ExistentialSolved(ty) => {
+  //           todo!()
+  //           // ContextEntry::ExistentialSolved(ty.bump_indexes(-1))
+  //         }
+  //         ContextEntry::Marker => ContextEntry::Marker,
+  //       })
+  //       .collect(),
+  //   }
+  // }
 }
 
 /// An index into the context.
@@ -507,3 +638,33 @@ pub fn convert_term(term: &data::Term) -> Term {
     term,
   )
 }
+
+pub trait Visitor {
+  fn visit_context_entry_before(&self, expr: &mut ContextEntry) -> Result<()> {
+    Ok(())
+  }
+  fn visit_context_entry_after(&self, expr: &mut ContextEntry) -> Result<()> {
+    Ok(())
+  }
+  fn visit_type_before(&self, expr: &mut Type) -> Result<()> {
+    Ok(())
+  }
+  fn visit_type_after(&self, expr: &mut Type) -> Result<()> {
+    Ok(())
+  }
+  fn visit_monotype_before(&self, expr: &mut Monotype) -> Result<()> {
+    Ok(())
+  }
+  fn visit_monotype_after(&self, expr: &mut Monotype) -> Result<()> {
+    Ok(())
+  }
+  fn visit_term_before(&self, expr: &mut Term) -> Result<()> {
+    Ok(())
+  }
+  fn visit_term_after(&self, expr: &mut Term) -> Result<()> {
+    Ok(())
+  }
+  fn visit_index(&self, index: &mut ContextIndex) -> Result<()> {
+    Ok(())
+  }
+}