These functions allow for the creation and manipulation of FITS HDU Groups, as defined in "A Hierarchical Grouping Convention for FITS" by Jennings, Pence, Folk and Schlesinger:
A group is a collection of HDUs whose association is defined by a grouping table. HDUs which are part of a group are referred to as member HDUs or simply as members. Grouping table member HDUs may themselves be grouping tables, thus allowing for the construction of open-ended hierarchies of HDUs.
Grouping tables contain one row for each member HDU. The grouping table columns provide identification information that allows applications to reference or "point to" the member HDUs. Member HDUs are expected, but not required, to contain a set of GRPIDn/GRPLCn keywords in their headers for each grouping table that they are referenced by. In this sense, the GRPIDn/GRPLCn keywords "link" the member HDU back to its Grouping table. Note that a member HDU need not reside in the same FITS file as its grouping table, and that a given HDU may be referenced by up to 999 grouping tables simultaneously.
Grouping tables are implemented as FITS binary tables with up to six pre-defined column TTYPEn values: 'MEMBER_XTENSION', 'MEMBER_NAME', 'MEMBER_VERSION', 'MEMBER_POSITION', 'MEMBER_URI_TYPE' and 'MEMBER_LOCATION'. The first three columns allow member HDUs to be identified by reference to their XTENSION, EXTNAME and EXTVER keyword values. The fourth column allows member HDUs to be identified by HDU position within their FITS file. The last two columns identify the FITS file in which the member HDU resides, if different from the grouping table FITS file.
Additional user defined "auxiliary" columns may also be included with any grouping table. When a grouping table is copied or modified the presence of auxiliary columns is always taken into account by the grouping support functions; however, the grouping support functions cannot directly make use of this data.
If a grouping table column is defined but the corresponding member HDU information is unavailable then a null value of the appropriate data type is inserted in the column field. Integer columns (MEMBER_POSITION, MEMBER_VERSION) are defined with a TNULLn value of zero (0). Character field columns (MEMBER_XTENSION, MEMBER_NAME, MEMBER_URI_TYPE, MEMBER_LOCATION) utilize an ASCII null character to denote a null field value.
The grouping support functions belong to two basic categories: those that work with grouping table HDUs (ffgt**) and those that work with member HDUs (ffgm**). Two functions, fits_copy_group() and fits_remove_group(), have the option to recursively copy/delete entire groups. Care should be taken when employing these functions in recursive mode as poorly defined groups could cause unpredictable results. The problem of a grouping table directly or indirectly referencing itself (thus creating an infinite loop) is protected against; in fact, neither function will attempt to copy or delete an HDU twice.