Random polynomial of degree 1

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Having defined

• the finite field K of size 4 in a
• a polynomial ring R1 over K

one can ask for a random element using the random_element method.

The random polynomial I obtained that way was of degree two:

sage: K.<a> = FiniteField(4)
sage: R1 = PolynomialRing(K, ['z%s' % p for p in range(1, 3)])
sage: R1.random_element()
(a)*z1*z2 + (a + 1)*z2^2 + z1 + (a)*z2

If I only want a random linear polynomial from this ring, how can I get one?