diff --git a/crates/coremem/src/stim/time_varying.rs b/crates/coremem/src/stim/time_varying.rs
index 30a0c02..e48be85 100644
--- a/crates/coremem/src/stim/time_varying.rs
+++ b/crates/coremem/src/stim/time_varying.rs
@@ -1,8 +1,7 @@
 //! time-varying portions of a Stimulus
 
 use crate::real::{self, Real};
-use crate::stim::{Fields, FieldMags, Stimulus};
-use crate::geom::Index;
+use crate::stim::FieldMags;
 
 pub trait TimeVarying<R> {
     fn at(&self, t_sec: R) -> FieldMags<R>;
@@ -179,13 +178,6 @@ impl<R: Real, T: TimeVarying<R>> TimeVarying<R> for Shifted<R, T> {
     }
 }
 
-// TODO: is this necessary?
-impl<R: Real, T: Stimulus<R>> Stimulus<R> for Shifted<R, T> {
-    fn at(&self, t_sec: R, feat_size: R, loc: Index) -> Fields<R> {
-        self.inner.at(t_sec - self.start_at, feat_size, loc)
-    }
-}
-
 
 #[derive(Clone)]
 pub struct Scaled<A, B>(A, B);