The sum p ( _q) is simply the number obtained by subtracting the absolute value of q from p, in the negative direction.
Mathematically, we can represent it as:
p ( _q) = p - |q|
For example, let's say p = 10 and q = 5. The distance |q| from p in the negative direction would be -5. Therefore, the sum p ( _5) would be 10 - 5 = 5.