Question: Multiple Nested IF Statements

I'm trying to create a series of if commands which will return a value based on some initial conditions.  I have 5 equations, x1, x2, x3, x4 and x5, which contain functions containing parameters m and n.


If the initial condition is "yes", then x1, x2 and x3 could be used, if no, then only x4 and x5 can be used. 


For the "yes" case, if m = 0, then only x2 can be used, and likewise, if n=0, only x3 applies.  Should m > 0, n > 0, then x1 applies.

For the "no" case, if m = 0, no equation applies, for n=0, x5 is to be used.  If m>0, n>0, x4 should be used.


I have tried using the elif command within an if statement with no success.  Any help would be appreciated.


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