Deriving an asymmetric logistic growth model

[liquidFuzz]

Hi, I'm wondering how the asymmetric logistic growth model is derived. My main interest is this version that I've implemented in python.

$$ growth(t) = d + \frac{a - d}{(1 + (x/c)^b)^g} $$

In particular, how is the asymmetry variable introduced?

Best,

Fuzz