IVC: attempt to write to define the real number of columns

And clean old not functional/drafty code

ivc/src/ivc/columns.rs

@@ -1,4 +1,4 @@
-use super::N_LIMBS_XLARGE;
+use super::{N_ADDITIONAL_WIT_COL_QUAD, N_LIMBS_XLARGE};
 use crate::poseidon_8_56_5_3_2::{
     bn254::{
         Column as IVCPoseidonColumn, NB_FULL_ROUND as IVC_POSEIDON_NB_FULL_ROUND,
@@ -330,11 +330,15 @@ pub enum IVCColumn {
 }
 
 impl ColumnIndexer for IVCColumn {
-    // This should be
-    //   const N_COL: usize = std::cmp::max(IVCPoseidonColumn::N_COL, FECColumn::N_COL);
-    // which is runtime-only expression..?
-    // 333 is not enough
-    const N_COL: usize = 600;
+    /// Number of columns used by the IVC circuit
+    /// It contains at least the columns used for Poseidon, and the columns used
+    /// to reduce to degree 2 the constraints of the circuit
+    /// FIXME: This can be improved by changing a bit the layer.
+    /// The reduction to degree 2 should happen in the gadgets to avoid adding
+    /// extra columns and leave sparse rows.
+    // We consider IVCPoseidonColumn::N_COL but it should be the maximum with
+    // the different gadgets at each row.
+    const N_COL: usize = IVCPoseidonColumn::N_COL + N_ADDITIONAL_WIT_COL_QUAD;
 
     fn to_column(self) -> Column {
         match self {