Although **system** is an optional parameter, **system**, **lattice**
and **group** parameters together must provide enough information to
properly identify the Bravais lattice of the cell. Redundant information
is fine, but conflicting information is always flagged as an error.

Example:<cell ... system="cubic"/>(no default) Allowed values:cubic,tetragonal,orthorhombic,hexagonal,monoclinic,triclinic(optional)

The R lattices describe the seven space groups belonging to the hexagonal system that are described as R groups, when using the standard Hermann-Mauguin symbols: R3 (146), R-3 (148), R32 (155), R3m (160), R3c (161), R-3m (166), R-3c (167).

Although **lattice** is an optional parameter, **system**, **lattice**
and **group** parameters together must provide enough information to
properly identify the Bravais lattice of the cell. Redundant information
is fine, but conflicting information is always flagged as an error.

Example:<cell ... lattice="P"/>(no default) Allowed values:P,I,F,C,R(optional)

Cells with an **hexagonal** system and a rombohedric **R** lattice
(corresponding to the seven R space groups when using the standard Hermann-Mauguin
symbols), are always represented using the hexagonal axes and the obverse setting,
when the chosen volume is **conventional**, and the rombohedric axes, when the
chosen volume is **primitive**. Cells with an **hexagonal** system and a
primitive **P** lattice (corresponding to all the other space groups from 143
to 194 that are not R) are always represented using full hexagonal prismas, when
the chosen volume is **conventional**, and one-third of the hexagonal prismas,
when the chosen volume is **primitive**.

Although **group** is an optional parameter, **system**, **lattice**
and **group** parameters together must provide enough information to
properly identify the Bravais lattice of the cell. Redundant information
is fine, but conflicting information is always flagged as an error.

Example:<cell ... group="1"/>(no default) Allowed values:1-230(optional)

For the hexagonal system, two lengths are required (**a**
or **b** and **c**). For the monoclinic system, three
lengths are required (**a** and **b** and **c**),
plus the angle around the axis b (**ac**).

For the triclinic system, all six parameters are required. Each angle must be smaller than the sum of the other two and must be larger than the absolute difference of the other two, otherwise an error is produced. Redundant information is fine, but conflicting information is always flagged as an error.

Example:<cell ... a="1.0" b="2.0" c="3.0" ab="50.0" bc="60.0" ac="70.0"/>(no default) Allowed values: positive real (required)