diff --git a/src/linalg/decompositions.rs b/src/linalg/decompositions.rs
index 47409224..3cc1a0f5 100644
--- a/src/linalg/decompositions.rs
+++ b/src/linalg/decompositions.rs
@@ -1,11 +1,36 @@
 use std::num::{Zero, Float};
-use na::DVec;
 use na::DMat;
 use traits::operations::Transpose;
-use traits::structure::ColSlice;
+use traits::structure::{ColSlice, Eye, Indexable};
 use traits::geometry::Norm;
 use std::cmp::min;
 
+/// Get the householder matrix corresponding to a reflexion to the hyperplane
+/// defined by `vec̀ . It can be a reflexion contained in a subspace.
+///
+/// # Arguments
+/// * `dim` - the dimension of the space the resulting matrix operates in
+/// * `start` - the starting dimension of the subspace of the reflexion
+/// * `vec` - the vector defining the reflection.
+fn householder_matrix<N: Float,
+                      Mat: Eye + Indexable<(uint, uint), N>,
+                      V: Indexable<uint, N>>
+                      (dim: uint, start: uint, vec: V) -> Mat {
+    let mut qk : Mat = Eye::new_identity(dim);
+    let stop = start + vec.shape();
+    assert!(stop <= dim);
+    for j in range(start, stop) {
+        for i in range(start, stop) {
+            unsafe {
+                let vv = vec.unsafe_at(i) * vec.unsafe_at(j);
+                let qkij = qk.unsafe_at((i, j));
+                qk.unsafe_set((i, j), qkij - vv - vv);
+            }
+        }
+    }
+    qk
+}
+
 /// QR decomposition using Householder reflections
 /// # Arguments
 /// * `m` - matrix to decompose
@@ -13,42 +38,26 @@ pub fn decomp_qr<N: Clone + Float>(m: &DMat<N>) -> (DMat<N>, DMat<N>) {
     let rows = m.nrows();
     let cols = m.ncols();
     assert!(rows >= cols);
-    let mut q : DMat<N> = DMat::new_identity(rows);
+    let mut q : DMat<N> = Eye::new_identity(rows);
     let mut r = m.clone();
 
-    let subtract_reflection = |vec: DVec<N>| -> DMat<N> {
-        // FIXME: we don't handle the complex case here
-        let mut qk : DMat<N> = DMat::new_identity(rows);
-        let start = rows - vec.at.len();
-        for j in range(start, rows) {
-            for i in range(start, rows) {
-                unsafe {
-                    let vv = vec.at_fast(i - start) * vec.at_fast(j - start);
-                    let qkij = qk.at_fast(i, j);
-                    qk.set_fast(i, j, qkij - vv - vv);
-                }
-            }
-        }
-        qk
-    };
-
     let iterations = min(rows - 1, cols);
 
     for ite in range(0u, iterations) {
         let mut v = r.col_slice(ite, ite, rows);
         let alpha =
-            if unsafe { v.at_fast(ite) } >= Zero::zero() {
+            if unsafe { v.unsafe_at(ite) } >= Zero::zero() {
                 -Norm::norm(&v)
             }
             else {
                 Norm::norm(&v)
             };
         unsafe {
-            let x = v.at_fast(0);
-            v.set_fast(0, x - alpha);
+            let x = v.unsafe_at(0);
+            v.unsafe_set(0, x - alpha);
         }
         let _ = v.normalize();
-        let qk = subtract_reflection(v);
+        let qk: DMat<N> = householder_matrix(rows, 0, v);
         r = qk * r;
         q = q * Transpose::transpose_cpy(&qk);
     }
diff --git a/src/na.rs b/src/na.rs
index 415eb8f2..6832fb18 100644
--- a/src/na.rs
+++ b/src/na.rs
@@ -40,7 +40,8 @@ pub use traits::{
     UniformSphereSample,
     AnyVec,
     VecExt,
-    ColSlice, RowSlice
+    ColSlice, RowSlice,
+    Eye
 };
 
 pub use structs::{
diff --git a/src/structs/dmat.rs b/src/structs/dmat.rs
index 8fac029c..40a3dea3 100644
--- a/src/structs/dmat.rs
+++ b/src/structs/dmat.rs
@@ -9,7 +9,7 @@ use traits::operations::ApproxEq;
 use std::mem;
 use structs::dvec::{DVec, DVecMulRhs};
 use traits::operations::{Inv, Transpose, Mean, Cov};
-use traits::structure::{Cast, ColSlice, RowSlice};
+use traits::structure::{Cast, ColSlice, RowSlice, Eye, Indexable};
 use std::fmt::{Show, Formatter, Result};
 
 #[doc(hidden)]
@@ -181,19 +181,19 @@ impl<N> DMat<N> {
 
 // FIXME: add a function to modify the dimension (to avoid useless allocations)?
 
-impl<N: One + Zero + Clone> DMat<N> {
+impl<N: One + Zero + Clone> Eye for DMat<N> {
     /// Builds an identity matrix.
     /// 
     /// # Arguments
     /// * `dim` - The dimension of the matrix. A `dim`-dimensional matrix contains `dim * dim`
     /// components.
     #[inline]
-    pub fn new_identity(dim: uint) -> DMat<N> {
+    fn new_identity(dim: uint) -> DMat<N> {
         let mut res = DMat::new_zeros(dim, dim);
 
         for i in range(0u, dim) {
             let _1: N  = One::one();
-            res.set(i, i, _1);
+            res.set((i, i), _1);
         }
 
         res
@@ -206,13 +206,16 @@ impl<N: Clone> DMat<N> {
         i + j * self.nrows
     }
 
+}
+
+impl<N: Clone> Indexable<(uint, uint), N> for DMat<N> {
     /// Changes the value of a component of the matrix.
     ///
     /// # Arguments
-    ///   * `row` - 0-based index of the line to be changed
-    ///   * `col` - 0-based index of the column to be changed
+    /// * `rowcol` - 0-based tuple (row, col)  to be changed
     #[inline]
-    pub fn set(&mut self, row: uint, col: uint, val: N) {
+    fn set(&mut self, rowcol: (uint,  uint), val: N) {
+        let (row, col) = rowcol;
         assert!(row < self.nrows);
         assert!(col < self.ncols);
 
@@ -222,7 +225,8 @@ impl<N: Clone> DMat<N> {
 
     /// Just like `set` without bounds checking.
     #[inline]
-    pub unsafe fn set_fast(&mut self, row: uint, col: uint, val: N) {
+    unsafe fn unsafe_set(&mut self, rowcol: (uint, uint), val: N) {
+        let (row, col) = rowcol;
         let offset = self.offset(row, col);
         *self.mij.as_mut_slice().unsafe_mut_ref(offset) = val
     }
@@ -230,20 +234,38 @@ impl<N: Clone> DMat<N> {
     /// Reads the value of a component of the matrix.
     ///
     /// # Arguments
-    /// * `row` - 0-based index of the line to be read
-    /// * `col` - 0-based index of the column to be read
+    /// * `rowcol` - 0-based tuple (row, col)  to be read
     #[inline]
-    pub fn at(&self, row: uint, col: uint) -> N {
+    fn at(&self, rowcol: (uint, uint)) -> N {
+        let (row, col) = rowcol;
         assert!(row < self.nrows);
         assert!(col < self.ncols);
-        unsafe { self.at_fast(row, col) }
+        unsafe { self.unsafe_at((row, col)) }
     }
 
     /// Just like `at` without bounds checking.
     #[inline]
-    pub unsafe fn at_fast(&self, row: uint, col: uint) -> N {
+    unsafe fn unsafe_at(&self, rowcol: (uint,  uint)) -> N {
+        let (row, col) = rowcol;
         (*self.mij.as_slice().unsafe_ref(self.offset(row, col))).clone()
     }
+
+    #[inline]
+    fn swap(&mut self, rowcol1: (uint, uint), rowcol2: (uint, uint)) {
+        let (row1, col1) = rowcol1;
+        let (row2, col2) = rowcol2;
+        let offset1 = self.offset(row1, col1);
+        let offset2 = self.offset(row2, col2);
+        let count = self.mij.len();
+        assert!(offset1 < count);
+        assert!(offset1 < count);
+        self.mij.as_mut_slice().swap(offset1, offset2);
+    }
+
+    fn shape(&self) -> (uint, uint) {
+        (self.nrows, self.ncols)
+    }
+
 }
 
 impl<N: Clone + Mul<N, N> + Add<N, N> + Zero> DMatMulRhs<N, DMat<N>> for DMat<N> {
@@ -258,10 +280,11 @@ impl<N: Clone + Mul<N, N> + Add<N, N> + Zero> DMatMulRhs<N, DMat<N>> for DMat<N>
 
                 unsafe {
                     for k in range(0u, left.ncols) {
-                        acc = acc + left.at_fast(i, k) * right.at_fast(k, j);
+                        acc = acc
+                            + left.unsafe_at((i, k)) * right.unsafe_at((k, j));
                     }
 
-                    res.set_fast(i, j, acc);
+                    res.unsafe_set((i, j), acc);
                 }
             }
         }
@@ -282,7 +305,7 @@ DMatMulRhs<N, DVec<N>> for DVec<N> {
 
             for j in range(0u, left.ncols) {
                 unsafe {
-                    acc = acc + left.at_fast(i, j) * right.at_fast(j);
+                    acc = acc + left.unsafe_at((i, j)) * right.unsafe_at(j);
                 }
             }
 
@@ -306,7 +329,7 @@ DVecMulRhs<N, DVec<N>> for DMat<N> {
 
             for j in range(0u, right.nrows) {
                 unsafe {
-                    acc = acc + left.at_fast(j) * right.at_fast(j, i);
+                    acc = acc + left.unsafe_at(j) * right.unsafe_at((j, i));
                 }
             }
 
@@ -335,7 +358,7 @@ Inv for DMat<N> {
         assert!(self.nrows == self.ncols);
 
         let dim              = self.nrows;
-        let mut res: DMat<N> = DMat::new_identity(dim);
+        let mut res: DMat<N> = Eye::new_identity(dim);
         let     _0T: N       = Zero::zero();
 
         // inversion using Gauss-Jordan elimination
@@ -347,7 +370,7 @@ Inv for DMat<N> {
             let mut n0 = k; // index of a non-zero entry
 
             while n0 != dim {
-                if unsafe { self.at_fast(n0, k) } != _0T {
+                if unsafe { self.unsafe_at((n0, k)) } != _0T {
                     break;
                 }
 
@@ -370,30 +393,30 @@ Inv for DMat<N> {
             }
 
             unsafe {
-                let pivot = self.at_fast(k, k);
+                let pivot = self.unsafe_at((k, k));
 
                 for j in range(k, dim) {
-                    let selfval = self.at_fast(k, j) / pivot;
-                    self.set_fast(k, j, selfval);
+                    let selfval = self.unsafe_at((k, j)) / pivot;
+                    self.unsafe_set((k, j), selfval);
                 }
 
                 for j in range(0u, dim) {
-                    let resval = res.at_fast(k, j) / pivot;
-                    res.set_fast(k, j, resval);
+                    let resval = res.unsafe_at((k, j)) / pivot;
+                    res.unsafe_set((k, j), resval);
                 }
 
                 for l in range(0u, dim) {
                     if l != k {
-                        let normalizer = self.at_fast(l, k);
+                        let normalizer = self.unsafe_at((l, k));
 
                         for j in range(k, dim) {
-                            let selfval = self.at_fast(l, j) - self.at_fast(k, j) * normalizer;
-                            self.set_fast(l, j, selfval);
+                            let selfval = self.unsafe_at((l, j)) - self.unsafe_at((k, j)) * normalizer;
+                            self.unsafe_set((l, j), selfval);
                         }
 
                         for j in range(0u, dim) {
-                            let resval = res.at_fast(l, j) - res.at_fast(k, j) * normalizer;
-                            res.set_fast(l, j, resval);
+                            let resval = res.unsafe_at((l, j)) - res.unsafe_at((k, j)) * normalizer;
+                            res.unsafe_set((l, j), resval);
                         }
                     }
                 }
@@ -422,7 +445,7 @@ impl<N: Clone> Transpose for DMat<N> {
             for i in range(0u, m.nrows) {
                 for j in range(0u, m.ncols) {
                     unsafe {
-                        res.set_fast(j, i, m.at_fast(i, j))
+                        res.unsafe_set((j, i), m.unsafe_at((i, j)))
                     }
                 }
             }
@@ -460,8 +483,8 @@ impl<N: Num + Cast<f32> + Clone> Mean<DVec<N>> for DMat<N> {
         for i in range(0u, m.nrows) {
             for j in range(0u, m.ncols) {
                 unsafe {
-                    let acc = res.at_fast(j) + m.at_fast(i, j) * normalizer;
-                    res.set_fast(j, acc);
+                    let acc = res.unsafe_at(j) + m.unsafe_at((i, j)) * normalizer;
+                    res.unsafe_set(j, acc);
                 }
             }
         }
@@ -482,7 +505,7 @@ impl<N: Clone + Num + Cast<f32> + DMatDivRhs<N, DMat<N>> + ToStr > Cov<DMat<N>>
         for i in range(0u, m.nrows) {
             for j in range(0u, m.ncols) {
                 unsafe {
-                    centered.set_fast(i, j, m.at_fast(i, j) - mean.at_fast(j));
+                    centered.unsafe_set((i, j), m.unsafe_at((i, j)) - mean.unsafe_at(j));
                 }
             }
         }
@@ -520,7 +543,7 @@ impl<N: Clone> RowSlice<DVec<N>> for DMat<N> {
         let mut slice_idx = 0u;
         for col_id in range(col_start, col_end) {
             unsafe {
-                slice.set_fast(slice_idx, self.at_fast(row_id, col_id));
+                slice.unsafe_set(slice_idx, self.unsafe_at((row_id, col_id)));
             }
             slice_idx += 1;
         }
@@ -553,7 +576,7 @@ impl<N: Show + Clone> Show for DMat<N> {
     fn fmt(&self, form:&mut Formatter) -> Result {
         for i in range(0u, self.nrows()) {
             for j in range(0u, self.ncols()) {
-                let _ = write!(form.buf, "{} ", self.at(i, j));
+                let _ = write!(form.buf, "{} ", self.at((i, j)));
             }
             let _ = write!(form.buf, "\n");
         }
diff --git a/src/structs/dvec.rs b/src/structs/dvec.rs
index b45a1a47..4da43229 100644
--- a/src/structs/dvec.rs
+++ b/src/structs/dvec.rs
@@ -9,7 +9,7 @@ use std::slice::{Items, MutItems};
 use traits::operations::ApproxEq;
 use std::iter::FromIterator;
 use traits::geometry::{Dot, Norm};
-use traits::structure::{Iterable, IterableMut};
+use traits::structure::{Iterable, IterableMut, Indexable};
 
 #[doc(hidden)]
 mod metal;
@@ -48,11 +48,42 @@ impl<N: Zero + Clone> DVec<N> {
     }
 }
 
-impl<N: Clone> DVec<N> {
-    /// Indexing without bounds checking.
-    pub unsafe fn at_fast(&self, i: uint) -> N {
+impl<N: Clone> Indexable<uint, N> for DVec<N> {
+
+    fn at(&self, i: uint) -> N {
+        assert!(i < self.at.len());
+        unsafe {
+            self.unsafe_at(i)
+        }
+    }
+
+    fn set(&mut self, i: uint, val: N) {
+        assert!(i < self.at.len());
+        unsafe {
+            self.unsafe_set(i, val);
+        }
+    }
+
+    fn swap(&mut self, i: uint, j: uint) {
+        assert!(i < self.at.len());
+        assert!(j < self.at.len());
+        self.at.as_mut_slice().swap(i, j);
+    }
+
+    fn shape(&self) -> uint {
+        self.at.len()
+    }
+
+    #[inline]
+    unsafe fn unsafe_at(&self, i: uint) -> N {
         (*self.at.as_slice().unsafe_ref(i)).clone()
     }
+
+    #[inline]
+    unsafe fn unsafe_set(&mut self, i: uint, val: N) {
+        *self.at.as_mut_slice().unsafe_mut_ref(i) = val
+    }
+
 }
 
 impl<N: One + Clone> DVec<N> {
@@ -86,11 +117,6 @@ impl<N> DVec<N> {
         }
     }
 
-    #[inline]
-    pub unsafe fn set_fast(&mut self, i: uint, val: N) {
-        *self.at.as_mut_slice().unsafe_mut_ref(i) = val
-    }
-
     /// Gets a reference to of this vector data.
     #[inline]
     pub fn as_vec<'r>(&'r self) -> &'r [N] {
@@ -261,7 +287,7 @@ impl<N: Num + Clone> Dot<N> for DVec<N> {
         let mut res: N = Zero::zero();
 
         for i in range(0u, a.at.len()) {
-            res = res + unsafe { a.at_fast(i) * b.at_fast(i) };
+            res = res + unsafe { a.unsafe_at(i) * b.unsafe_at(i) };
         }
 
         res
@@ -272,7 +298,7 @@ impl<N: Num + Clone> Dot<N> for DVec<N> {
         let mut res: N = Zero::zero();
 
         for i in range(0u, a.at.len()) {
-            res = res + unsafe { (a.at_fast(i) - b.at_fast(i)) * c.at_fast(i) };
+            res = res + unsafe { (a.unsafe_at(i) - b.unsafe_at(i)) * c.unsafe_at(i) };
         }
 
         res
diff --git a/src/structs/mat.rs b/src/structs/mat.rs
index c17831cf..0d180fcf 100644
--- a/src/structs/mat.rs
+++ b/src/structs/mat.rs
@@ -9,7 +9,8 @@ use std::slice::{Items, MutItems};
 use structs::vec::{Vec1, Vec2, Vec3, Vec4, Vec5, Vec6, Vec1MulRhs, Vec4MulRhs,
                    Vec5MulRhs, Vec6MulRhs};
 
-use traits::structure::{Cast, Row, Col, Iterable, IterableMut, Dim, Indexable};
+use traits::structure::{Cast, Row, Col, Iterable, IterableMut, Dim, Indexable,
+                        Eye};
 use traits::operations::{Absolute, Transpose, Inv, Outer};
 use traits::geometry::{ToHomogeneous, FromHomogeneous};
 
@@ -34,6 +35,8 @@ pub struct Mat1<N> {
     pub m11: N
 }
 
+eye_impl!(Mat1, 1, m11)
+
 double_dispatch_binop_decl_trait!(Mat1, Mat1MulRhs)
 double_dispatch_binop_decl_trait!(Mat1, Mat1DivRhs)
 double_dispatch_binop_decl_trait!(Mat1, Mat1AddRhs)
@@ -127,6 +130,8 @@ pub struct Mat2<N> {
     pub m12: N, pub m22: N
 }
 
+eye_impl!(Mat2, 2, m11, m22)
+
 double_dispatch_binop_decl_trait!(Mat2, Mat2MulRhs)
 double_dispatch_binop_decl_trait!(Mat2, Mat2DivRhs)
 double_dispatch_binop_decl_trait!(Mat2, Mat2AddRhs)
@@ -225,6 +230,8 @@ pub struct Mat3<N> {
     pub m13: N, pub m23: N, pub m33: N
 }
 
+eye_impl!(Mat3, 3, m11, m22, m33)
+
 double_dispatch_binop_decl_trait!(Mat3, Mat3MulRhs)
 double_dispatch_binop_decl_trait!(Mat3, Mat3DivRhs)
 double_dispatch_binop_decl_trait!(Mat3, Mat3AddRhs)
@@ -337,6 +344,8 @@ pub struct Mat4<N> {
     pub m14: N, pub m24: N, pub m34: N, pub m44: N
 }
 
+eye_impl!(Mat4, 4, m11, m22, m33, m44)
+
 double_dispatch_binop_decl_trait!(Mat4, Mat4MulRhs)
 double_dispatch_binop_decl_trait!(Mat4, Mat4DivRhs)
 double_dispatch_binop_decl_trait!(Mat4, Mat4AddRhs)
@@ -501,6 +510,8 @@ pub struct Mat5<N> {
     pub m15: N, pub m25: N, pub m35: N, pub m45: N, pub m55: N
 }
 
+eye_impl!(Mat5, 5, m11, m22, m33, m44, m55)
+
 double_dispatch_binop_decl_trait!(Mat5, Mat5MulRhs)
 double_dispatch_binop_decl_trait!(Mat5, Mat5DivRhs)
 double_dispatch_binop_decl_trait!(Mat5, Mat5AddRhs)
@@ -681,6 +692,8 @@ pub struct Mat6<N> {
     pub m16: N, pub m26: N, pub m36: N, pub m46: N, pub m56: N, pub m66: N
 }
 
+eye_impl!(Mat6, 6, m11, m22, m33, m44, m55, m66)
+
 double_dispatch_binop_decl_trait!(Mat6, Mat6MulRhs)
 double_dispatch_binop_decl_trait!(Mat6, Mat6DivRhs)
 double_dispatch_binop_decl_trait!(Mat6, Mat6AddRhs)
diff --git a/src/structs/mat_macros.rs b/src/structs/mat_macros.rs
index 2c13926d..6d08435b 100644
--- a/src/structs/mat_macros.rs
+++ b/src/structs/mat_macros.rs
@@ -98,6 +98,20 @@ macro_rules! scalar_add_impl(
     )
 )
 
+
+macro_rules! eye_impl(
+    ($t: ident, $ndim: expr, $($comp_diagN: ident),+) => (
+        impl<N: Zero + One> Eye for $t<N> {
+            fn new_identity(dim: uint) -> $t<N> {
+                assert!(dim == $ndim);
+                let mut eye: $t<N> = Zero::zero();
+                $(eye.$comp_diagN = One::one();)+
+                eye
+            }
+        }
+    )
+)
+
 macro_rules! scalar_sub_impl(
     ($t: ident, $n: ident, $trhs: ident, $comp0: ident $(,$compN: ident)*) => (
         impl $trhs<$n, $t<$n>> for $n {
@@ -193,6 +207,11 @@ macro_rules! indexable_impl(
             }
         }
 
+        #[inline]
+        fn shape(&self) -> (uint, uint) {
+            ($dim, $dim)
+        }
+
         #[inline]
         unsafe fn unsafe_at(&self, (i, j): (uint, uint)) -> N {
             (*cast::transmute::<&$t<N>, &[N, ..$dim * $dim]>(self).unsafe_ref(i + j * $dim)).clone()
diff --git a/src/structs/spec/vec0.rs b/src/structs/spec/vec0.rs
index 4843e3e1..ce493dda 100644
--- a/src/structs/spec/vec0.rs
+++ b/src/structs/spec/vec0.rs
@@ -25,6 +25,11 @@ impl<N> Indexable<uint, N> for vec::Vec0<N> {
     fn set(&mut self, _: uint, _: N) {
     }
 
+    #[inline]
+    fn shape(&self) -> uint {
+        0
+    }
+
     #[inline]
     fn swap(&mut self, _: uint, _: uint) {
     }
diff --git a/src/structs/vec_macros.rs b/src/structs/vec_macros.rs
index 0eeafb93..90e5ca9f 100644
--- a/src/structs/vec_macros.rs
+++ b/src/structs/vec_macros.rs
@@ -165,6 +165,11 @@ macro_rules! indexable_impl(
                 }
             }
 
+            #[inline]
+            fn shape(&self) -> uint {
+                $dim
+            }
+
             #[inline]
             fn swap(&mut self, i1: uint, i2: uint) {
                 unsafe {
diff --git a/src/traits/mod.rs b/src/traits/mod.rs
index 9c774d80..1c471b45 100644
--- a/src/traits/mod.rs
+++ b/src/traits/mod.rs
@@ -6,7 +6,7 @@ pub use self::geometry::{AbsoluteRotate, Cross, CrossMatrix, Dot, FromHomogeneou
 
 pub use self::structure::{FloatVec, FloatVecExt, Basis, Cast, Col, Dim, Indexable,
                           Iterable, IterableMut, Mat, Row, AnyVec, VecExt,
-                          ColSlice, RowSlice};
+                          ColSlice, RowSlice, Eye};
 
 pub use self::operations::{Absolute, ApproxEq, Cov, Inv, LMul, Mean, Outer, PartialOrd, RMul,
                            ScalarAdd, ScalarSub, Transpose};
diff --git a/src/traits/structure.rs b/src/traits/structure.rs
index 953f13b9..0e94e6ad 100644
--- a/src/traits/structure.rs
+++ b/src/traits/structure.rs
@@ -19,6 +19,12 @@ pub trait Mat<R, C> : Row<R> + Col<C> + RMul<R> + LMul<C> { }
 impl<M: Row<R> + Col<C> + RMul<R> + LMul<C>, R, C> Mat<R, C> for M {
 }
 
+/// Trait for constructing the identity matrix
+pub trait Eye {
+    /// Return the identity matrix of specified dimension
+    fn new_identity(dim: uint) -> Self;
+}
+
 // XXX: we keep ScalarAdd and ScalarSub here to avoid trait impl conflict (overriding) between the
 // different Add/Sub traits. This is _so_ unfortunate…
 
@@ -126,6 +132,9 @@ pub trait Indexable<Index, Res> {
     /// Swaps the `i`-th element of `self` with its `j`-th element.
     fn swap(&mut self, i: Index, j: Index);
 
+    /// Returns the shape of the iterable range
+    fn shape(&self) -> Index;
+
     /// Reads the `i`-th element of `self`.
     ///
     /// `i` is not checked.