/usr/include/coin/CoinPresolveDoubleton.hpp is in coinor-libcoinutils-dev 2.6.4-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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// Copyright (C) 2002, International Business Machines
// Corporation and others.  All Rights Reserved.
#ifndef CoinPresolveDoubleton_H
#define CoinPresolveDoubleton_H
#define	DOUBLETON	5
/*! \class doubleton_action
    \brief Solve ax+by=c for y and substitute y out of the problem.
  This moves the bounds information for y onto x, making y free and allowing
  us to substitute it away.
  \verbatim
	   a x + b y = c
	   l1 <= x <= u1
	   l2 <= y <= u2	==>
	  
	   l2 <= (c - a x) / b <= u2
	   b/-a > 0 ==> (b l2 - c) / -a <= x <= (b u2 - c) / -a
	   b/-a < 0 ==> (b u2 - c) / -a <= x <= (b l2 - c) / -a
  \endverbatim
*/
class doubleton_action : public CoinPresolveAction {
 public:
  struct action {
    double clox;
    double cupx;
    double costx;
    
    double costy;
    double rlo;
    double coeffx;
    double coeffy;
    double *colel;
    int icolx;
    int icoly;
    int row;
    int ncolx;
    int ncoly;
  };
  const int nactions_;
  const action *const actions_;
 private:
  doubleton_action(int nactions,
		      const action *actions,
		      const CoinPresolveAction *next) :
    CoinPresolveAction(next),
    nactions_(nactions), actions_(actions)
{}
 public:
  const char *name() const { return ("doubleton_action"); }
  static const CoinPresolveAction *presolve(CoinPresolveMatrix *,
					 const CoinPresolveAction *next);
  
  void postsolve(CoinPostsolveMatrix *prob) const;
  ~doubleton_action();
};
#endif
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