cross-refecence header file

Hi there! I've been struggling to find a solution to make a piece of code wo
rk. Basically I have a source structure and I've been asked to implement the
 functions. The problem is that there are two header files that cross-refere
nce each other and I have no clue about how to get this thing to compile:

code:

/*********** File clause.h ***********/
#ifndef CLAUSE_H_
#define CLAUSE_H_

#include "solver.h"

struct clause {
    bool learnt;
    int size;
    float activity;
    lit *lits;
};

typedef struct clause clause;

extern bool new_clause(solver *s, bool appr, float act, vec_i *v, clause *c)
;
#endif


code:

/***** clause.c  *****/
#include "clause.h"

bool new_clause(solver *s, bool appr, float act, vec_i *v, clause *c)
{
    /* DO THE JOB ;-) */
}


code:

/***** solver.h ****/

#ifndef SOLVER_H_
#define SOLVER_H_

struct solver {
    int of_clauses;
    clause *listOfClauses;
    /* many other fields... */
};

extern int solver_addclause(solver *, clause *);

#endif /* SOLVER_H_ */


code:

/****#include "solver.h"

int solver_addclause(solver *, clause *)
{
  /* DO THE JOB */  
}



Of course, if I try to compile this structure I get error messages because s
olver.h(which is pre-processed after clause.h) can't see the clause structur
e(because of the #ifndef guards).

Is there a way to make this thing compile correctly?

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