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       1                 :            : /******************************************************************************
       2                 :            :  * Top contributors (to current version):
       3                 :            :  *   Andrew Reynolds
       4                 :            :  *
       5                 :            :  * This file is part of the cvc5 project.
       6                 :            :  *
       7                 :            :  * Copyright (c) 2009-2025 by the authors listed in the file AUTHORS
       8                 :            :  * in the top-level source directory and their institutional affiliations.
       9                 :            :  * All rights reserved.  See the file COPYING in the top-level source
      10                 :            :  * directory for licensing information.
      11                 :            :  * ****************************************************************************
      12                 :            :  *
      13                 :            :  * Implementation of witness elimination node conversion
      14                 :            :  */
      15                 :            : #include "cvc5_private.h"
      16                 :            : 
      17                 :            : #ifndef CVC5__EXPR__ELIM_WITNESS_NODE_CONVERTER_H
      18                 :            : #define CVC5__EXPR__ELIM_WITNESS_NODE_CONVERTER_H
      19                 :            : 
      20                 :            : #include <unordered_set>
      21                 :            : 
      22                 :            : #include "expr/node.h"
      23                 :            : #include "expr/node_converter.h"
      24                 :            : #include "smt/env_obj.h"
      25                 :            : 
      26                 :            : namespace cvc5::internal {
      27                 :            : 
      28                 :            : /**
      29                 :            :  * Node converter to eliminate all terms of kind WITNESS. Each term replaced
      30                 :            :  * in this way is captured by a skolem that witnesses the axiom for that
      31                 :            :  * witness.
      32                 :            :  *
      33                 :            :  * Witness terms are required to track their justification as part of their
      34                 :            :  * AST. In particular, it is required that all terms of kind WITNESS are given
      35                 :            :  * an instantiation attribute of the form:
      36                 :            :  *   (INST_ATTRIBUTE "witness" (SEXPR r a1 ... an))
      37                 :            :  * where r is the (integer value of) a proof rule and a1...an are arguments
      38                 :            :  * to that proof rule. This instantiation attribute is always constructed
      39                 :            :  * assuming that ValidWitnessProofGenerator
      40                 :            :  * (proof/valid_witness_proof_generator.h) is used to construct the witness
      41                 :            :  * terms. These annotations are expected to be robust to rewriting and
      42                 :            :  * substitution, e.g. rewriting (SEXPR r a1 ... an) does not change whether
      43                 :            :  * it is a valid input to the definition of a proof rule. (Note this is not the
      44                 :            :  * case for ProofRule::EXISTS_INV_CONDITION, which is why
      45                 :            :  * ProofRule::MACRO_EXISTS_INV_CONDITION is used internally).
      46                 :            :  *
      47                 :            :  * For each witness of this form, we replace the witness by its corresponding
      48                 :            :  * skolem and collect its corresponding axiom, defining what lemma we can
      49                 :            :  * assume about it, which can be retrieved via ::getAxioms in this class.
      50                 :            :  *
      51                 :            :  * Note that we use WITNESS terms for two reasons:
      52                 :            :  * (1) (witness x (= x t)) can naturally rewrite to t, which we wish to
      53                 :            :  * infer when applicable by substitution + rewriting.
      54                 :            :  * (2) witness terms trigger this class to recognize when axioms should be
      55                 :            :  * added as lemmas. In other words, at the moment witness terms are
      56                 :            :  * eliminated, we ensure their axiom is recorded as well.
      57                 :            :  */
      58                 :            : class ElimWitnessNodeConverter : protected EnvObj, public NodeConverter
      59                 :            : {
      60                 :            :  public:
      61                 :            :   /** Eliminate witness terms.*/
      62                 :            :   ElimWitnessNodeConverter(Env& env);
      63                 :        765 :   ~ElimWitnessNodeConverter() {}
      64                 :            :   /**
      65                 :            :    * Convert node n as described above during post-order traversal.
      66                 :            :    */
      67                 :            :   Node postConvert(Node n) override;
      68                 :            :   /**
      69                 :            :    * Get the axioms
      70                 :            :    */
      71                 :            :   const std::vector<Node>& getAxioms() const;
      72                 :            :   /**
      73                 :            :    * Get the normal form of a quantified formula for which we are introducing
      74                 :            :    * a skolem variable based on eliminating a witness term.
      75                 :            :    */
      76                 :            :   virtual Node getNormalFormFor(const Node& q);
      77                 :            : 
      78                 :            :  private:
      79                 :            :   /** The list of axioms introduced by eliminating witness */
      80                 :            :   std::vector<Node> d_axioms;
      81                 :            : };
      82                 :            : 
      83                 :            : }  // namespace cvc5::internal
      84                 :            : 
      85                 :            : #endif