How to create in Sage system of equations using elements from boolean field?

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I want to create a field on 8 elements, and then, using this elements make a System of equations like:

nb = 8
varl = [c+ str(p) for c in 'xy' for p in range (nb)]
B = BooleanPolynomialRing(names = varl)

f1 = x1 + x7*x2

f2 = x4*x6*x8 + x7

and then....

But in this case Sage give me an error NameError: name 'x1' is not defined

And in this case:

f1 = x[3] + x[1]*x[2]
f2 = x[4]*x[6]*x[2] + x[7]

error: TypeError: 'sage.symbolic.expression.Expression' object does not support indexing

Is there any way to create them in Sage?