diff --git a/timely/src/dataflow/operators/capability.rs b/timely/src/dataflow/operators/capability.rs
index 5caa9bce8..72c872350 100644
--- a/timely/src/dataflow/operators/capability.rs
+++ b/timely/src/dataflow/operators/capability.rs
@@ -34,9 +34,10 @@ use crate::scheduling::Activations;
 use crate::dataflow::channels::pullers::counter::ConsumedGuard;
 
 /// An internal trait expressing the capability to send messages with a given timestamp.
-pub trait CapabilityTrait<T: Timestamp> {
+pub trait CapabilityTrait<T: Timestamp>: private::Sealed {
     /// The timestamp associated with the capability.
     fn time(&self) -> &T;
+    /// Checks if this capability is valid for the output port associated with `query_buffer`
     fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool;
 }
 
@@ -53,6 +54,18 @@ impl<'a, T: Timestamp, C: CapabilityTrait<T>> CapabilityTrait<T> for &'a mut C {
     }
 }
 
+mod private {
+    use crate::progress::Timestamp;
+
+    pub trait Sealed {}
+
+    impl<'a, C: Sealed> Sealed for &'a C { }
+    impl<'a, C: Sealed> Sealed for &'a mut C { }
+    impl<T: Timestamp> Sealed for super::Capability<T> { }
+    impl<T: Timestamp> Sealed for super::InputCapability<T> { }
+    impl<T: Timestamp> Sealed for super::ActivateCapability<T> { }
+}
+
 /// The capability to send data with a certain timestamp on a dataflow edge.
 ///
 /// Capabilities are used by timely dataflow's progress tracking machinery to restrict and track
diff --git a/timely/src/dataflow/operators/mod.rs b/timely/src/dataflow/operators/mod.rs
index 508d10ac6..a8c00c401 100644
--- a/timely/src/dataflow/operators/mod.rs
+++ b/timely/src/dataflow/operators/mod.rs
@@ -60,6 +60,5 @@ pub mod generic;
 pub mod reclock;
 pub mod count;
 
-// keep "mint" module-private
-mod capability;
+pub mod capability;
 pub use self::capability::{ActivateCapability, Capability, InputCapability, CapabilitySet, DowngradeError};