message: Improve naming in distiller
diff: @@ -13,8 +13,8 @@ const PLACEHOLDER_RANGE: ByteRange = ByteRange::new(PLACEHOLDER_POS, PLACEHOLDER
  pub struct Context<'arena> {
  /// Scoped arena for storing distilled terms.
  scope: &'arena Scope<'arena>,
  - /// Name environment.
  - names: UniqueEnv<StringId>,
  + /// Rigid name environment.
  + rigid_names: UniqueEnv<StringId>,
  placeholder_string: StringId,
  }
  @@ -26,31 +26,31 @@ impl<'arena> Context<'arena> {
  Context {
  scope,
  - names: UniqueEnv::new(),
  + rigid_names: UniqueEnv::new(),
  placeholder_string,
  }
  }
  fn rigid_len(&mut self) -> EnvLen {
  - self.names.len()
  + self.rigid_names.len()
  }
  fn get_rigid_name(&self, var: LocalVar) -> Option<StringId> {
  - self.names.get_local(var).copied()
  + self.rigid_names.get_local(var).copied()
  }
  fn push_rigid(&mut self, name: Option<StringId>) -> StringId {
  - let name = name.unwrap_or(self.placeholder_string); // TODO: choose a better name
  - self.names.push(name); // TODO: ensure we chose a correctly bound name
  + let name = name.unwrap_or(self.placeholder_string); // TODO: choose a better name?
  + self.rigid_names.push(name); // TODO: ensure we chose a correctly bound name
  name
  }
  fn pop_rigid(&mut self) {
  - self.names.pop();
  + self.rigid_names.pop();
  }
  fn truncate_rigid(&mut self, len: EnvLen) {
  - self.names.truncate(len);
  + self.rigid_names.truncate(len);
  }
  /// Distill a core term into a surface term, in a 'checkable' context.
  @@ -124,7 +124,7 @@ impl<'arena> Context<'arena> {
  match info {
  core::EntryInfo::Concrete => {}
  core::EntryInfo::Abstract => {
  - let var = self.names.len().global_to_local(var).unwrap();
  + let var = self.rigid_len().global_to_local(var).unwrap();
  let input_expr = self.synth(&core::Term::RigidVar(var));
  head_expr = Term::FunElim(
  self.scope.to_scope(head_expr),
  