Need to be DPP

[r-barnes]

This isn't DPP

def cp_log_ratio_norm(a, b):
    return cp.maximum(a / b, b * cp.inv_pos(a))

specifically, a / b is not DPP.

@AlexLipp

[r-barnes]

From this doc I see:

As another example, the quotient expr / p is not DPP-compliant when p is a parameter, but this can be rewritten as expr * p_tilde, where p_tilde is a parameter that represents 1/p.

I'll try to incorporate this wisdom.

[r-barnes]

This is fixed in #21