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General principle of the M6Geo geometry
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The geometry is defined between the keywords ``GEOM`` and ``ENDG``.
It is composed of volumes, each defined by:

-	a shape (specified using the ``TYPE`` keyword) chosen from a predefined set;
-	a position within the module;
-	relationships with other volumes in the module.


The primary component of the geometry is the module, which is delimited by the ``MODU`` and ``ENDM`` keywords.
Modules are volume combinations that can be used in several locations within the geometry without needing to repeat their definition.
Each module can be thought of as a "local" geometry with its own:

-	coordinate system;
-	types and volumes;
-	external volume.

Modules can be inserted into each other using holes. Every volume must be contained within a module.

Modules are sequenced in the geometry as follows:

.. code-block:: m6geo
  :caption: Block syntax

  GEOM

    MODU 0  * Module 0 is always required
      * module description
    ENDM
      * module description
    MODU id
      * module description
    ENDM
  ENDG


The description of a modular geometry is governed by the following rules:

- Modules can be described in any order within the ``GEOM`` block;
-	The main module ``MODU 0`` is always required;
-	The external volume of the main module (i.e. ``MODU 0``) serves as the external volume of the global geometry;
-	The external volume of a module encompasses all volumes related to the module;
- The external volume must be enclosed (TYPE of type ``BOX``, ``SPHE``, ``ELLI``, ``CYL(X|Y|Z)``, ``HEX(X|Y|Z)``, or ``MPLA`` keywords);
- The external volume must not use geometrical operators.

.. note::
  The names of holes, volumes, and types are independent (i.e. a type may share the same name as a volume).