partly improve coercions over constant lines

still needs a real quality check, or something, for stuff like
e : (x ≡ x : A) ⊢ coe (𝑖 ⇒ e @𝑖) x

lib/Quox/Syntax/Term/Tighten.idr

@@ -314,6 +314,19 @@ dsqueeze (S names (Y body)) =
 dsqueeze (S names (N body)) = S names $ N body
 
 
+export
+squeezed : {s : Nat} -> ScopeTermN s d n ->
+           Either (BContext s, Term d (s + n)) (Term d n)
+squeezed (S ns (N t)) = Right t
+squeezed (S ns (Y t)) = maybe (Left (ns, t)) Right $ tightenN s t
+
+export
+dsqueezed : {s : Nat} -> DScopeTermN s d n ->
+            Either (BContext s, Term (s + d) n) (Term d n)
+dsqueezed (S ns (N t)) = Right t
+dsqueezed (S ns (Y t)) = maybe (Left (ns, t)) Right $ tightenN s t @{TermD}
+
+
 
 public export %inline
 CompH' : (ty : DScopeTerm d n) -> (p, q : Dim d) -> (val : Term d n) ->
@@ -324,7 +337,6 @@ CompH' {ty, p, q, val, r, zero, one, loc} =
   Comp {
     ty = dsub1 ty q, p, q,
     val = E $ Coe ty p q val val.loc, r,
-    -- [fixme] better locations for these vars?
     zero = DST zero.names $ E $
       Coe ty' (B VZ zero.loc) (weakD 1 q) zero.term zero.loc,
     one = DST one.names $ E $