symbolic variables and +=

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When I substitute symbolic variables into a function, the sum notation "+=" does not allow them because it only accepts reals. I need to substitute symbolic variables into the function. Is there any workaround? (I have tried sum(), but that breaks my function.)

Here's my attempt to sub in symbolic variables:

x1,x2 = var('x1 x2')

A2 = Afunction(2, [x1,x2])