a real drawback, since our rules involve sets. Part of this difficulty comes from a second drawback: In ASP, it is hard to reuse portions of a program. Similar rules should be written again, in a different way. Also, "brave" or "cautious" reasoning is generally not allowed (except with respect to precise "queries"). In ASP, "the problem is the program" and the "solution" consists in one **or several**
sets of atoms satisfying the problem. Each such set is an answer set**. Brave and** cautious solutions mean to look for atoms true respectively in some or in all the answer sets.

This complicates the use of ASP: any modification becomes complex. However, things are evolving, e.g. DLV-Complex (www.mat.unical.it/dlv-complex),
deals with the data structure problem and DLT [3] allows the use of "templates", convenient for reusing part of a program. We present the explanation formalism, then its ASP translation in DLV-Complex, and we conclude by a few reasonable expectations about the future ASP systems which could help a final **user.**

## 2 The Causal Explanation Formalism 2.1 Preliminaries **(Propositional Version, Cf [2] For The Full Formalism).**

We distinguish various types of statements:
C: A theory expressing causal statements. E.g. On alarm causes Heard **bell**.

O: IS-A **links between items which can appear in a causal statement.E.g.,**
T emperature 39 →IS−A F ever **T emperature**, Heard sof t bell →IS−A Heard **bell**.

W: A classical propositional theory expressing truths (incompatible facts, cooccurring facts, . . .). E.g., Heard sof t bell → ¬Heard loud **bell**.
Propositional symbols denote states of affairs, which can be "facts" or "events" such as F ever T emperature or On alarm**. The causal statements express**
causal relations between facts or events.

Some care is necessary when providing these causal and ontological atoms. If
"Flu causes F ever T emperature", we conclude Flu explains **T emperature** 39 from T emperature 39 →IS−A F ever T emperature**, but we cannot state**
Flu causes T emperature **39: the causal information must be "on the right**
level".

The formal system infers formulas denoting explanations from C ∪ O ∪ W.

The IS A atoms express knowledge necessary to infer explanations. In the following, α, β, . . . denote the propositional atoms and Φ, Ψ, . . . **denote sets thereof.**
Atoms 1. Propositional atoms: **α, β, . . .**.

2. Causal atoms: **α causes β**.