WTF Factorial

Def: WTF Factorial
Denoted by: N?!
Pronounced: N what the phuque factorial.

Denote the smallest positive prime that's greater than and not equal to N by{N} p,
and N?! := (N/{N}p)*((N-1)/{N-1}p)*((N-2)/{N-2}p)*...*(1/2).
eg. 4?! = (4/5)*(3/5)*(2/3)*(1/2).

NB: by definition, N?!<1