Add tentative fix for #357

Author: cschwan

pineappl/src/grid.rs

@@ -944,17 +944,8 @@ impl Grid {
                     .multi_slice_mut((s![.., .., index], s![.., .., other_index]));
 
                 for (lhs, rhs) in a.iter_mut().zip(b.iter_mut()) {
-                    if !rhs.is_empty() {
-                        if lhs.is_empty() {
-                            // we can't merge into an EmptySubgridV1
-                            *lhs = mem::replace(rhs, EmptySubgridV1.into());
-                            // transpose `lhs`
-                            todo!();
-                        } else {
-                            lhs.merge(rhs, Some((a_subgrid, b_subgrid)));
-                            *rhs = EmptySubgridV1.into();
-                        }
-                    }
+                    lhs.merge(rhs, Some((a_subgrid, b_subgrid)));
+                    *rhs = EmptySubgridV1.into();
                 }
             }
         }

pineappl/src/subgrid.rs

@@ -418,15 +418,21 @@ impl SubgridEnum {
         if other.is_empty() {
             return;
         }
+
         if let Self::EmptySubgridV1(_) = self {
-            if transpose.is_none() {
+            if transpose.is_some() {
+                // change the type of `self` to the type of `other`
                 *self = other.clone();
+                // TODO: emptying `self` could be done more efficiently, we're probably storing a
+                // lot of zeros here
+                self.scale(0.0);
             } else {
-                todo!();
+                *self = other.clone();
+                return;
             }
-        } else {
-            self.merge_impl(other, transpose);
         }
+
+        self.merge_impl(other, transpose);
     }
 }