I think that, given the definition of dependence I have proposed, the second premise is clear. If X depends on X1 then in order for X to exist X1 must exist. If X1 depends on X2 then, in order for X1 to exist X2 must exist. This process will continue indefinitely and, if each Xi is dependent on some Xi+1 for it’s existence then there will always be a further cause (or depended-upon-thing) to take into account and X will never come into existence. X will only come into existence if there exists in this chain of dependency some Xn which depends on nothing else for it’s existence.
An infinite regression of dependencies does not an existence cause.
Yoda probably said that.