Rename operation::chain to then

... and make `focus_*` operations generic over
the output type.

core/src/widget/operation.rs

@@ -295,7 +295,7 @@ where
 
 /// Chains the output of an [`Operation`] with the provided function to
 /// build a new [`Operation`].
-pub fn chain<A, B, O>(
+pub fn then<A, B, O>(
     operation: impl Operation<A> + 'static,
     f: fn(A) -> O,
 ) -> impl Operation<B>
@@ -361,7 +361,7 @@ where
                     Outcome::Chain(Box::new((self.next)(value)))
                 }
                 Outcome::Chain(operation) => {
-                    Outcome::Chain(Box::new(chain(operation, self.next)))
+                    Outcome::Chain(Box::new(then(operation, self.next)))
                 }
             }
         }

core/src/widget/operation/focusable.rs

@@ -94,7 +94,10 @@ pub fn count() -> impl Operation<Count> {
 /// Produces an [`Operation`] that searches for the current focused widget, and
 /// - if found, focuses the previous focusable widget.
 /// - if not found, focuses the last focusable widget.
-pub fn focus_previous() -> impl Operation {
+pub fn focus_previous<T>() -> impl Operation<T>
+where
+    T: Send + 'static,
+{
     struct FocusPrevious {
         count: Count,
         current: usize,
@@ -128,13 +131,16 @@ pub fn focus_previous() -> impl Operation {
         }
     }
 
-    operation::chain(count(), |count| FocusPrevious { count, current: 0 })
+    operation::then(count(), |count| FocusPrevious { count, current: 0 })
 }
 
 /// Produces an [`Operation`] that searches for the current focused widget, and
 /// - if found, focuses the next focusable widget.
 /// - if not found, focuses the first focusable widget.
-pub fn focus_next() -> impl Operation {
+pub fn focus_next<T>() -> impl Operation<T>
+where
+    T: Send + 'static,
+{
     struct FocusNext {
         count: Count,
         current: usize,
@@ -162,7 +168,7 @@ pub fn focus_next() -> impl Operation {
         }
     }
 
-    operation::chain(count(), |count| FocusNext { count, current: 0 })
+    operation::then(count(), |count| FocusNext { count, current: 0 })
 }
 
 /// Produces an [`Operation`] that searches for the current focused widget