How to find a Lyapunov function? $$\dot{x} = x - x^2 - xy$$
$$\dot{y} = y - y^2 - xy$$
Is there a general method to find a Lyapunov function? Why do I feel like finding a Lyapunov function is like shooting in the dark and luck-dependent? I tried to guess some function with $x$ and $y$, but it doesn't work. I also tried Mathematica, and apparently, this is too complicated for Mathematica to solve. 
A side-note question: if the general method to find a Lyapunov function is currently not available, is that because no one is smart enough to come up with the general method? Or is it due to its nature that it's "impossible" to have the general method? And has anyone proved its impossibility? 
Note: it's not a duplicate question because only my side-note question is a duplicate of the other question. My first question is tailored to this specific system, which is unique. 

Actually there is no straightforward method to calculate Lyapunov function but you should care that we compute Lyapunov function to show the stability of a dynamic system about its equilibrium point, where the derivative of the variables are equal to zero. In this case  $(x,y)=(1,0) , (0,0)$. You should try to find a Lapunov function
