On page 2 of this pdf the author introduces this equivalence relation:
$$f(x) \sim_{x_0} g(x) \iff f(x) - g(x) = o(x-x_0)\quad \text{as } x\to x_0$$
Assuming this is a standard notation, how does one read $f(x) \sim_{x_0} g(x)$? As in, what should the blank be in "f of x is [blank] g of x"?