diff --git a/include/mapper.h b/include/mapper.h
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+/** @file mapper.h
+ *  @brief Definitions for mapper objects.
+ *
+ * A mapper objects defines how and what to map between two grid families.
+ * For example, a mapper object can be used to map the charge from
+ * a @a cdr grid to a @a Poisson grid, or to map the @a potential of a Poisson
+ * grid into one of the components of the @a electric field in a @a cdr grid.
+ * For that, a mapper object has to implement some methods that tell us
+ * how to map things in the cases\n
+ * a) the source grid is coarser than the target grid (interpolation),\n
+ * b) the source and the target are at the same level (copy),\n
+ * c) the target grid is coarser (coarsening).
+ */
+
+#ifndef _MAPPER_H_
+
+typedef struct mapper_t mapper_t;
+
+#ifndef _INTERPOL2_H_
+# include "interpol2.h"
+#endif
+
+#define _coarsen_params_ (mapper_t *mapper, grid_t *source, grid_t *target, \
+			  int ir, int iz, int itheta)
+#define _copy_params_ (mapper_t *mapper, grid_t *source, grid_t *target, \
+		       int ir, int iz, int itheta)
+#define _interpol_set_params_ (mapper_t *mapper, grid_t *source, \
+			       interpol_t *interpol, int ir, \
+			       int iz, int itheta)
+#define _interpol_params_ (mapper_t *mapper, grid_t *source, grid_t *target, \
+			   interpol_t *interpol, int ir, int iz, int itheta)
+
+
+struct mapper_t {
+  interpol_method_t *interpol_method;
+
+  void (*coarsen) _coarsen_params_;
+  void (*copy) _copy_params_;
+  /* may return false if we are outside the source grid. */
+  int (*interpol_set) _interpol_set_params_;
+
+  void (*interpol) _interpol_params_;
+
+  /* This is used for the interpolation of electric fields and indicates
+   * the staggering of the cells.  When we call the interpolator to set
+   * the stencil at coordinates pr, pz, the fine-grid cells whose value will
+   * later be calculated ir, iz with
+   *   pr << level_diff + shift_r << (level_diff - 1) <= ir 
+   * < pr << level_diff + shift_r << (level_diff - 1)
+   * 
+   * (and the equivalent for iz). 
+   * Hence note that for interpolation in not-staggered grids, 
+   * (i.e. charge, densities, ...)
+   * shift_r = shift_z = 0.
+   */
+  int shift_r, shift_z;
+
+  /* We provide this extra member to allow the easy creation of different
+   * mappers that share the same functions but have a slightly different
+   * behaviour.  In particular this is used in cdr.c to define
+   * different interpolators for each species.  In that case, extra is the
+   * species index.
+   */
+  int extra;
+};
+
+
+/** Usually, we will define functions called xxxx_coarsen, xxxx_copy, etc
+   to implement the above interface.  We define these macros to facilitate
+   the declaration of such functions. */
+#define decl_mapper_funcs(_VAR)				\
+  void _VAR ## _coarsen _coarsen_params_;		\
+  void _VAR ## _copy _copy_params_;			\
+  int _VAR ## _interpol_set _interpol_set_params_;	\
+  void _VAR ## _interpol _interpol_params_
+
+/** Useful to init a staggered mapper object */
+#define mk_mapper_staggered(_C, _INTERPOL_METHOD, _SHIFT_R, _SHIFT_Z)	\
+  {_INTERPOL_METHOD, _C ## _coarsen, _C ## _copy,			\
+      _C ## _interpol_set, _C ##_interpol, _SHIFT_R, _SHIFT_Z, 0}
+
+/** Useful to init a mapper object */
+#define mk_mapper(_C, _INTERPOL_METHOD)					\
+  mk_mapper_staggered (_C, _INTERPOL_METHOD, 0, 0)
+
+/** Mappers that only copy or interpolate. */
+#define mk_mapper_down(_C, _INTERPOL_METHOD)				\
+  {_INTERPOL_METHOD, NULL, _C ## _copy,					\
+      _C ## _interpol_set, _C ##_interpol, 0, 0, 0}
+
+#define _MAPPER_H_
+#endif