Are they keywords or system variables.?

>Run this example:
>

>#define INP
>#define REF
>#define OUT
>
>? INP REF OUT && Variable 'REF' is not found.
>? INP REF     && Variable 'REF' is not found.
>? INP
>
>? INP OUT REF && Variable 'OUT' is not found.
>? INP OUT     && Variable 'OUT' is not found.
>

>
>
>Why preprocesor ignoring each second constant at compiling?
>
>MartinJ