How do I stop maple from running the rest of the worksheet in case an error occurs in the middle of it? Which command should I use?

More specifically, I have a program in which I input the masses of the particles and then the program will calculate the corresponding parameters of the potential, and will do a lot of other stuff with these parameters. However, it these parameters happen to be complex, the program should simply halt with a warning: "your parameters are complex, cannot continue".

I have a 6x6 matrix that depends on 8 parameters, M(F) (F is an 8-dim array, say...), and I want to compute the eigenvalues of this matrix both at a fixed point F_0 and at some other arbitrary point F'. The problem is that I need to make sure the ordering of the Eigenvalues remains the same at these two points, so that if I'd go continuously from F_0 to F' the eigenvalues would go continuously from Eigenvalues(M(F_0)) to Eigenvalues(M(F')), without any flip in the order of the entries.