7.3} Description of the geometry

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 The Combinatorial Geometry used by FLUKA is based on two important concepts:
 bodies and regions. The first ones are closed solid bodies (spheres,
 parallelepipeds, etc.) or semi-infinite portions of space (half-spaces,
 infinite cylinders) delimited by surfaces of first or second degree. The user
 must combine bodies by boolean operations (addition, intersection and
 subtraction) to perform a complete partition of the space of interest into
 regions, namely cells of uniform material composition.  One important rule to
 remember is that inside the space of interest, defined by means of an external
 closed body, every point must belong to one and only one region.

 Input for the geometry description, which has its own format and rules,
 explained in Chap. 9, must be contained between a GEOBEGIN and a GEOEND card.
 These two cards follow the normal FLUKA input syntax. An option offered by the
 GEOBEGIN command is to read the geometry input from a separate file. Command
 GEOEND can be used also to invoke the geometry debugger, a check which is
 always strongly recommended.

 Geometry input, sandwiched between a GEOBEGIN and a GEOEND card can be placed
 anywhere in the input file (before the START command). It is mandatory in all
 cases.

 An optional command related to geometry is PLOTGEOM. It is used to display
 sections of the geometry and needs to read its own input, as explained later.

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