how can you make 1 particle allow another inside it? i imagine its in graphics.c

/*
RETURN-value explenation
1 = Swap
0 = No move/Bounce
2 = Both particles occupy the same space.
*/
int eval_move(int pt, int nx, int ny, unsigned *rr)
{
unsigned r;

if (nx<0 || ny<0 || nx>=XRES || ny>=YRES)
return 0;

r = pmap[ny][nx];
if (r && (r>>8) r = (r&~0xFF) | parts[r>>8].type;
if (rr)
*rr = r;

if ((r&0xFF)==PT_VOID || (r&0xFF)==PT_BHOL)
return 1;

if (pt==PT_PHOT&&(
(r&0xFF)==PT_GLAS || (r&0xFF)==PT_PHOT ||
(r&0xFF)==PT_CLNE || (r&0xFF)==PT_PCLN ||
(r&0xFF)==PT_GLOW || (r&0xFF)==PT_WATR ||
(r&0xFF)==PT_DSTW || (r&0xFF)==PT_SLTW ||
(r&0xFF)==PT_ISOZ || (r&0xFF)==PT_ISZS ||
(r&0xFF)==PT_FILT || (r&0xFF)==PT_INVIS ||
(r&0xFF)==PT_QRTZ || (r&0xFF)==PT_PQRT ||
((r&0xFF)==PT_LCRY&&parts[r>>8].life > 5)))
return 2;

k, i notice this for photoelectric effect do i have to create a new one for my element and instead off naming the element do i just use parts [i]