invalid directive foreign(...

Hi,

I have a project in which I need to convert a date to a unique number
and vice versa. For simplicity reasons, the unique number is the number
of days since 1.1.1970.

I have found some C code and changed it according to my needs. It was
working nicely when testing it in a C main programme.

However, when I modeled the predicate to be later used in the project on
the example in the manual and compiled it with gplc ("gplc date_unix.pl
date_unix.c"), I got a message "invalid directive
foreign(date_unix(?stamp,?year,?month,?day))."

I cannot find my mistake, is there anything that I did not spot when
writing the predicate? I have attached the sources as a tar file, I hope
this is OK.

Cheers Christoph

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