Need operator, x || y = x*y/(x+y)

[FunMod]

I know how to define a function but, I need a operator. It would be great to have the operator || when I use it for parallel resistance calcualtion.This parallel resistance operator is not to speed up the calculation in computer but to simply the result in low entropy symbolic format for the reading and understanding of human.
x||y=1/(1/x+1/y)
x||y=1/(1/x+1/y+1/z)