Computing $\int_0^1\int_0^{1-x}f(x,y)\:{\rm d}y\:{\rm d}x$ using MATLAB

Question

I want to compute $$\int_0^1\int_0^{1-x}f(x,y)\:{\rm d}y\:{\rm d}x$$ numerically using MATLAB R2012b. I wasn't able to figure out how I need to deal with the dependency on the first variable in the inner integral.

So, how do I need to do this?