add FromVarR, etc

`FromVarR` is a version of `FromVar` which needs the scope size
to be relevant at runtime

lib/Quox/Syntax/Term/Subst.idr

@@ -60,9 +60,31 @@ namespace DSubst.DScopeTermN
   S ns (N body) // th = S ns $ N $ body // th
 
 
-export %inline FromVar (Elim d) where fromVar = B
-export %inline FromVar (Term d) where fromVar = E .: fromVar
+export %inline
+FromVarR (Elim d) where fromVarR = B
 
+export %inline
+FromVar (Elim d) where fromVar = B; fromVarSame _ _ = Refl
+
+export %inline
+FromVarR (Term d) where fromVarR = E .: fromVarR
+
+export %inline
+FromVar (Term d) where fromVar = E .: fromVar; fromVarSame _ _ = Refl
+
+
+export
+CanSubstSelf (Elim d)
+
+private
+tsubstElim : Elim d from -> Lazy (TSubst d from to) -> Elim d to
+tsubstElim (F x u loc)       _  = F x u loc
+tsubstElim (B i loc)         th = get th i loc
+tsubstElim (CloE (Sub e ph)) th = assert_total CloE $ Sub e $ ph . th
+tsubstElim e                 th =
+  case force th of
+       Shift SZ => e
+       th       => CloE $ Sub e th
 
 ||| does the minimal reasonable work:
 ||| - deletes the closure around a *free* name
@@ -70,14 +92,10 @@ export %inline FromVar (Term d) where fromVar = E .: fromVar
 ||| - composes (lazily) with an existing top-level closure
 ||| - immediately looks up a bound variable
 ||| - otherwise, wraps in a new closure
+CanSubstSelfR (Elim d) where (//?) = tsubstElim
+
 export
-CanSubstSelf (Elim d) where
-  F x u loc       // _  = F x u loc
-  B i loc         // th = get th i loc
-  CloE (Sub e ph) // th = assert_total CloE $ Sub e $ ph . th
-  e               // th = case force th of
-                               Shift SZ => e
-                               th       => CloE $ Sub e th
+CanSubstSelf (Elim d) where (//) = tsubstElim; substSame _ _ = Refl
 
 namespace CanTSubst
   public export