diff --git a/src/nested/map/fits/write.rs b/src/nested/map/fits/write.rs
index df8bc74..9181d99 100644
--- a/src/nested/map/fits/write.rs
+++ b/src/nested/map/fits/write.rs
@@ -22,7 +22,7 @@ pub fn write_implicit_skymap_fits<R: Write, T: SkyMapValue>(
   // since we already know it.
   // In the end, using monomorphisation, the binary code is duplicated so...
   let n_cells = values.len() as u64;
-  let depth = depth_from_n_hash_unsafe(values.len());
+  let depth = depth_from_n_hash_unsafe(values.len() as u64);
   if n_cells != n_hash(depth) {
     return Err(FitsError::new_custom(format!(
       "Number of cell {} not compatible with an HEALPix depth of {}. Expected: {}.",
diff --git a/src/nested/map/mom/impls/zvec.rs b/src/nested/map/mom/impls/zvec.rs
index 4170cfe..105f7ff 100644
--- a/src/nested/map/mom/impls/zvec.rs
+++ b/src/nested/map/mom/impls/zvec.rs
@@ -204,6 +204,52 @@ where
     }
     Self { depth, entries }
   }
+
+  fn merge<'s, L, R, S, O, M>(lhs: L, rhs: R, split: S, op: O, merge: M) -> Self
+  where
+    L: Mom<'s, ZUniqHType = Z, ValueType = V>,
+    R: Mom<'s, ZUniqHType = Z, ValueType = V>,
+    S: Fn(
+      Self::ValueType,
+    ) -> (
+      Self::ValueType,
+      Self::ValueType,
+      Self::ValueType,
+      Self::ValueType,
+    ),
+    O: Fn(Option<Self::ValueType>, Option<Self::ValueType>) -> Option<Self::ValuesIt>,
+    M: Fn(
+      Self::ValueType,
+      Self::ValueType,
+      Self::ValueType,
+      Self::ValueType,
+    ) -> Result<
+      Self::ValueType,
+      (
+        Self::ValueType,
+        Self::ValueType,
+        Self::ValueType,
+        Self::ValueType,
+      ),
+    >,
+  {
+    struct DHZV<ZZ, VV> {
+      d: u8,
+      h: ZZ,
+      z: ZZ,
+      v: VV,
+    }
+    let zv_to_dhzv = |(z, v)| {
+      let (d, h) = Self::ZUniqHType::from_zuniq(z);
+      DHZV { d, h, z, v }
+    };
+    let mut stack: Vec<(Z, V)> = Vec::with_capacity(lhs.len() + rhs.len());
+    let mut it_left = lhs.owned_entries();
+    let mut it_right = rhs.owned_entries();
+    let mut left = it_left.next().map(zv_to_dhzv);
+    let mut right = it_right.next().map(zv_to_dhzv);
+    todo!()
+  }
 }
 
 impl<Z, V> MomVecImpl<Z, V>
diff --git a/src/nested/map/mom/mod.rs b/src/nested/map/mom/mod.rs
index 19aba13..fe8c645 100644
--- a/src/nested/map/mom/mod.rs
+++ b/src/nested/map/mom/mod.rs
@@ -231,18 +231,19 @@ pub trait Mom<'a>: Sized {
       M: Fn(Self::ValueType, Self::ValueType, Self::ValueType, Self::ValueType) -> Result<Self::ValueType, (Self::ValueType, Self::ValueType, Self::ValueType, Self::ValueType)>,
       Self::ValueType: 's;
   
-  fn merge<L, R, S, O, M>(lhs: L, rhs: R, split: S, op: O, merge: M) -> Self
+  fn merge<'s, L, R, S, O, M>(lhs: L, rhs: R, split: S, op: O, merge: M) -> Self
     where
+      L: Mom<'s, ZUniqHType = Self::ZUniqHType, ValueType = Self::ValueType>,
+      R: Mom<'s, ZUniqHType = Self::ZUniqHType, ValueType = Self::ValueType>,
       // Split a parent cell into for siblings
       S: Fn(Self::ValueType) -> (Self::ValueType, Self::ValueType, Self::ValueType, Self::ValueType),
       // Merge the values of a same cell from both MOMs
-      O: Fn(Self::ValueType, Self::ValueType) -> Self::ValuesIt,
-      // Performs a post-merge operation?
-      M: Fn(Self::ValueType, Self::ValueType, Self::ValueType, Self::ValueType) -> Self::ValueType,
-  {
-    // get both owned iterators
-    todo!()
-  }
+      // or decide what to do is one of the MOM has a value and the other does not
+      // (the (None, None) must never be called).
+      // This allow for various types of JOIN (inner, left, right, full).
+      O: Fn(Option<Self::ValueType>, Option<Self::ValueType>) -> Option<Self::ValuesIt>,
+      // Possibly performs a post-merge operation.
+      M: Fn(Self::ValueType, Self::ValueType, Self::ValueType, Self::ValueType) -> Result<Self::ValueType, (Self::ValueType, Self::ValueType, Self::ValueType, Self::ValueType)>;
 
   
 }
diff --git a/src/nested/map/skymap.rs b/src/nested/map/skymap.rs
index bfa9f34..c37f619 100644
--- a/src/nested/map/skymap.rs
+++ b/src/nested/map/skymap.rs
@@ -172,14 +172,14 @@ impl<'a, H: HHash, V: SkyMapValue + 'a> SkyMap<'a> for ImplicitSkyMapArray<H, V>
     self.depth
   }
 
-  fn len(&self) -> usize {
-    self.values.len()
-  }
-
   fn is_implicit(&self) -> bool {
     true
   }
 
+  fn len(&self) -> usize {
+    self.values.len()
+  }
+
   fn get(&self, hash: Self::HashType) -> &Self::ValueType {
     &self.values.deref()[hash.as_()]
   }
@@ -207,6 +207,73 @@ impl<'a, H: HHash, V: SkyMapValue + 'a> SkyMap<'a> for ImplicitSkyMapArray<H, V>
   }
 }
 
+pub struct ImplicitSkyMapArrayRef<'a, H: HHash, V: SkyMapValue> {
+  depth: u8,
+  values: &'a [V],
+  _htype: PhantomData<H>,
+}
+impl<'a, H: HHash, V: SkyMapValue + 'a> ImplicitSkyMapArrayRef<'a, H, V> {
+  /// WARNING: we assume that the coherency between the depth and the number of elements in the
+  ///array has already been tested.
+  pub fn new(depth: u8, values: &'a [V]) -> Self {
+    assert_eq!(
+      n_hash(depth) as usize,
+      values.deref().len(),
+      "Wrong implicit skymap size. Epecgted: {}. Actual: {}.",
+      n_hash(depth),
+      values.len()
+    );
+    Self {
+      depth,
+      values,
+      _htype: PhantomData,
+    }
+  }
+}
+impl<'a, H: HHash, V: SkyMapValue + Clone + 'a> SkyMap<'a> for ImplicitSkyMapArrayRef<'a, H, V> {
+  type HashType = H;
+  type ValueType = V;
+  type ValuesIt = Iter<'a, Self::ValueType>;
+  type EntriesIt = Map<Enumerate<Self::ValuesIt>, fn((usize, &V)) -> (H, &V)>;
+  type OwnedEntriesIt = Map<Enumerate<Self::ValuesIt>, fn((usize, &V)) -> (H, V)>;
+
+  fn depth(&self) -> u8 {
+    self.depth
+  }
+
+  fn is_implicit(&self) -> bool {
+    true
+  }
+
+  fn len(&self) -> usize {
+    self.values.len()
+  }
+
+  fn get(&self, hash: Self::HashType) -> &Self::ValueType {
+    &self.values.deref()[hash.as_()]
+  }
+
+  fn values(&'a self) -> Self::ValuesIt {
+    self.values.deref().iter()
+  }
+
+  fn entries(&'a self) -> Self::EntriesIt {
+    self
+      .values
+      .iter()
+      .enumerate()
+      .map(move |(h, v)| (H::from_usize(h), v))
+  }
+
+  fn owned_entries(self) -> Self::OwnedEntriesIt {
+    self
+      .values
+      .iter()
+      .enumerate()
+      .map(move |(h, v)| (H::from_usize(h), v.clone()))
+  }
+}
+
 pub enum SkyMapEnum {
   ImplicitU64U8(ImplicitSkyMapArray<u64, u8>),
   ImplicitU64I16(ImplicitSkyMapArray<u64, i16>),