2 files changed, 35 insertions, 10 deletions

diff --git a/rprt-engine/src/selection.rs b/rprt-engine/src/selection.rs
index ee83294..1d1c557 100644
--- a/rprt-engine/src/selection.rs
+++ b/rprt-engine/src/selection.rs
@@ -75,15 +75,6 @@ where
     SelectionError(SelectionError),
 }
 
-impl<E> From<E> for VectoriseError<E>
-where
-    E: std::error::Error + 'static,
-{
-    fn from(e: E) -> Self {
-        VectoriseError::ProcessingError(e)
-    }
-}
-
 impl Selection {
     pub fn empty() -> Self {
         Self::MultiRanges {
diff --git a/rprt-engine/src/selection_function.rs b/rprt-engine/src/selection_function.rs
index 3da61c9..0b6e98e 100644
--- a/rprt-engine/src/selection_function.rs
+++ b/rprt-engine/src/selection_function.rs
@@ -47,7 +47,37 @@ impl From<SFComponents> for SelectionFunction<VectoriseError<SFError>> {
 }
 
 impl SelectionFunction<VectoriseError<SFError>> {
-    pub fn empty() -> Self {
+    pub fn transform_conditional(sf: Self) -> Self {
+        fn condition(
+            pred: Result<Selection, VectoriseError<SFError>>,
+            res: Selection,
+        ) -> Result<Selection, VectoriseError<SFError>> {
+            match pred {
+                Ok(mut sel) => Ok(if sel.is_empty() { sel } else { res }),
+                Err(e) => Err(e),
+            }
+        }
+
+        let dyadic: Option<Box<dyn Fn(&EditorState, &Selection, &Selection) -> Result<_, _>>> =
+            if let Some(dyadic) = sf.dyadic {
+                Some(Box::new(move |es, left, right| {
+                    condition((dyadic)(es, left, right), left.clone())
+                }))
+            } else {
+                None
+            };
+
+        let monadic: Box<dyn Fn(&EditorState, &Selection) -> Result<_, _>> =
+            Box::new(move |es, left| condition((sf.monadic)(es, left), left.clone()));
+
+        SelectionFunction {
+            niladic: sf.niladic,
+            monadic,
+            dyadic,
+        }
+    }
+    // ----------------------------------------------------------------
+    pub fn empty_sequential() -> Self {
         SFComponents {
             niladic: Box::new(|_| Ok(Selection::empty())),
             monadic_rank0: Box::new(|_, _, _| Ok(Selection::empty())),
@@ -57,6 +87,10 @@ impl SelectionFunction<VectoriseError<SFError>> {
         .into()
     }
 
+    pub fn empy_structural() -> Self {
+        Self::empty_sequential()
+    }
+
     pub fn end_sequential() -> Self {
         fn get_end_position(es: &EditorState, buffer_id: BufferID) -> Result<Selection, SFError> {
             if let Some(buf) = es.get_buffer(buffer_id) {