Logic gate tier rating

S:

NAND

A:

NOR

B:

NOT, AND

C:

OR

D:

E:

XOR

F:

XNOR

>>5
#define bool_merge2_(a,b) a##b
#define bool_merge2(a,b) bool_merge2_(a,b)

What is the point of this? Why define two macros where the second one just calls the first?
I see a similar pattern too often but I have no idea what to google to figure it out.