Hall space group symbol (property)

{
    "$id": "https://schemas.optimade.org/defs/v1.2/properties/optimade/structures/space_group_symbol_hall",
    "$schema": "https://schemas.optimade.org/meta/v1.2/optimade/property_definition.json",
    "title": "Hall space group symbol",
    "x-optimade-type": "string",
    "x-optimade-definition": {
        "label": "space_group_symbol_hall_optimade_structures",
        "kind": "property",
        "version": "1.2.0",
        "format": "1.2",
        "name": "space_group_symbol_hall"
    },
    "type": [
        "string",
        "null"
    ],
    "description": "A Hall space group symbol representing the symmetry of the structure as defined in Hall, S. R. (1981), Acta Cryst. A37, 517-525 and erratum (1981), A37, 921.\n\n**Requirements/Conventions**:\n\n- The change-of-basis operations are used as defined in the International Tables of Crystallography (ITC) Vol. B, Sect. 1.4, Appendix A1.4.2 (IUCr, 2001).\n- Each component of the Hall symbol MUST be separated by a single space symbol.\n- If there exists a standard Hall symbol which represents the symmetry it SHOULD be used.\n- MUST be `null` if `nperiodic_dimensions` is not equal to 3.\n\n**Explained examples**:\n\n- Space group symbols with explicit origin (the Hall symbols):\n\n    - `P 2c -2ac`\n    - `-I 4bd 2ab 3`\n\n- Space group symbols with change-of-basis operations:\n\n    - `P 2yb (-1/2*x+z,1/2*x,y)`\n    - `-I 4 2 (1/2*x+1/2*y,-1/2*x+1/2*y,z)`\n\n**Bibliographic References**\n\n- Hall, S. R. (1981) Space-group notation with an explicit origin. Acta Crystallographica Section A, 37, 517-525, International Union of Crystallography (IUCr), DOI: https://doi.org/10.1107/s0567739481001228\n- Hall, S. R. (1981a) Space-group notation with an explicit origin; erratum. Acta Crystallographica Section A, 37, 921-921, International Union of Crystallography (IUCr), DOI: https://doi.org/10.1107/s0567739481001976\n- IUCr (2001). International Tables for Crystallography vol. B. Reciprocal Space. Ed. U. Shmueli. 2-nd edition. Dordrecht/Boston/London, Kluwer Academic Publishers.",
    "examples": [
        "P 2c -2ac",
        "-I 4db 2ab 3",
        "P 2yb (-1/2*x+z,1/2*x,y)",
        "-I 4 2 (1/2*x+1/2*y,-1/2*x+1/2*y,z)"
    ],
    "x-optimade-unit": "unapplicable"
}