Entity Generics Question

[dohzer]

I'm declaring the following interface and getting the following error:

entity entity_a is
	generic 
	(
		SIG_A_WIDTH 		: integer := 16;
		SIG_B_WIDTH 		: integer := 2;
		SIG_C_WIDTH		: integer := SIG_WIDTH_A - SIG_WIDTH_B;
	);

	port 
	(
		...
		blah
		...
	);
end entity_a;

Object 'sig_a_width' cannot be used within the same interface as it is declared

I want to be able to define "SIG_C_WIDTH" as a function of the other two widths, so is there an easier and/or more correct way to do it? Should I be declaring it in a separate package or somewhere else in the code?