The Dual Nature of NILNIL is a very fundamental piece of data. It has a dual nature, being both a symbol and a cons pair of the form (NIL . NIL). From the beginning, this has been a basic design requirement of PicoLisp.
As shown in "doc64/structures" (similar also in "doc/structures"):
NIL: / | V +-----+-----+-----+-----+ |'LIN'| / | / | / | +-----+--+--+-----+-----+Physically, the pointer to NIL (shown with the 'V' in the above diagram) is a true symbol, as it points at an offset of a pointer size into the memory heap. Taken as such, NIL is a normal symbol,
NIL: / | V +-----+-----+ |'LIN'| / | +-----+--+--+having a tail with the characters 'N', 'I' and 'L' (the name), and a value which in turn points to NIL (symbolized with the '/').
However, when this pointer is boldly used as a cell pointer (by ignoring the pointer tags caused by the pointer size offset)
NIL: / | V +-----+-----+ | / | / | +--+--+-----+then it is a normal cons pair, with NIL in its CAR and NIL in its CDR, which just happens to be misaligned in memory (i.e. at the place of a symbol).
This structure has great advantages. Any proper list (ending with NIL) becomes sort of "infinite", allowing to take the CDR as often as possible and still obtain NIL again and again.
Therefore, a function doesn't need to check whether it actually received an argument or not. It can simply take the next argument with CDR from the argument list, and doesn't see any difference between (foo NIL) and (foo). This makes interpretation both simpler and faster.