Calculating Groebner base with constants

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I want to see what the Groebner base of an ideal is, but the polynomials generating it have some constants inside. Is there any way to do that with Sage?

var('a')
R.<x,y> = PolynomialRing(CC, 2, order='lex')
I = ideal(y^4*a, y^2*x - y*a - x, y - x*a)
B = I.groebner_basis()
B

This doesn't seem to work. a is the constant. Also, this example is just a dummy, I want to know in general how to have constant coefficient in polynomials.