We now turn to a rather special form of Mapping, the CmpMap. The
Mappings we have considered so far have been atomic, in the sense that
they perform pre-defined elementary transformations. A CmpMap,
however, is a compound Mapping. In essence, it is a framework for
containing other Mappings and its purpose is to allow those Mappings
to work together in various combinations while appearing as a single
Object. A CmpMap's behaviour is therefore not pre-defined, but is
determined by the other Mappings it contains.
AST A Library for Handling World Coordinate Systems in Astronomy