Operates upon an input FITS file by modifying the text of commentary keywords in the header of the primary array or an extension.
New text provided by the user replaces the original text.
If the new text is too long to fit on a single line, additional lines of the same keyword type are inserted. The user must have write privilege to this file. Text which is matched against all header records of the specified keyword type to identify the line to be modified. Only the first letter is required, case insensitive.