add DimEqs.get etc

src/Quox/Syntax/DimEq.idr

@@ -39,10 +39,22 @@ new : (d : Nat) -> DimEq d
 new d = C (new' d)
 
 
-export
+private %inline
+shiftMay : Maybe (Dim from) -> Shift from to -> Maybe (Dim to)
+shiftMay p by = map (// by) p
+
+export %inline
+get' : DimEq' d -> Var d -> Maybe (Dim d)
+get' = getWith shiftMay
+
+private %inline
+getShift' : Shift len out -> DimEq' len -> Var len -> Maybe (Dim out)
+getShift' = getShiftWith shiftMay
+
+export %inline
 get : DimEq' d -> Dim d -> Dim d
 get _   (K e) = K e
-get eqs (B i) = fromMaybe (B i) $ (eqs !! i) @{Compose}
+get eqs (B i) = fromMaybe (B i) $ get' eqs i
 
 
 export %inline