Header (resp. startup file) is a piece of code what is run at the
beginning of the executable. It usually makes some variable
initiallisations, library opening etc. Each header should call
function called 'main()'. It is usually written in assembler to get
the best perfomance, but it can be written in PowerD as good.
Some example startup codes are in 'd:lib' directory.
If you wrote a header, you should write also a module what contains
in the header initialised but external variables (via 'EDEF') and
OPT HEAD='xxx.o' where xxx is the header object file name in 'd:lib',
or '*' and full path.