Function lam

crates/kernel/src/whnf.rs:3521–3523

(a: AE, b: AE)

      if args.len() != 2 {
        return Ok(None);
      }
      return self.try_reduce_string_dec_eq(e, &args);
    }

    let is_back = *addr == self.prims.string_back.addr
      || *addr == self.prims.string_legacy_back.addr;