Function head_const_id

crates/kernel/src/def_eq.rs:1643–1655

Extract head constant KId from expression or app spine.

(e: &KExpr<M>)

  fn dump_eta_trace(
    &self,
    reason: &str,
    id: Option<&KId<M>>,
    idx: usize,
    a: &KExpr<M>,
    b: &KExpr<M>,
  ) {
    let Some(filter) = IX_ETA_TRACE.as_ref() else {
      return;
    };
    if !self.debug_label_matches_env() {
      return;
    }
    let id_s = id.map_or_else(|| "<none>".into(), |id| id.to_string());
    if !filter.is_empty() && !id_s.contains(filter) {
      return;