Formal Template Syntax

The template syntax can formally be defined as follows:

    TEMPLATE = BLOCK [ BLOCK ... ]

       BLOCK = { HDU | GROUP }

       GROUP = \GROUP [ BLOCK ... ] \END

         HDU = XTENSION [ LINE ... ] { XTENSION | \GROUP | \END | EOF }

        LINE = [ KEYWORD [ = ] ] [ VALUE ] [ / COMMENT ]

    X ...     - X can be present 1 or more times
    { X | Y } - X or Y
    [ X ]     - X is optional

At the topmost level, the template defines 1 or more template blocks. Blocks can be either HDU (Header Data Unit) or a GROUP. For each block the parser creates 1 (or more for GROUPs) FITS file HDUs.