CAS-> Variable() warning msg?

[Han]

You should be careful with using functional notation on expressions as opposed to actual functions (and by functions, I mean as "defined" by the CAS).

a:=x^2;
x:={1,2,3,4};

a(1) through a(4) will behave as expected, but a(-1) will produce an error. The CAS is able to handle list processing, so that x^2 is {1,4,9,16}. The a(n) notation is now a reference to the n-th object in the list x^2. So a(-1) makes absolutely no sense in this context.