Declaring constants when working with Groebner bases [closed]

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I want to find the Groebner base of a ideal,the ideal is generated by some polynomials with constant coefficients, but they do not have numerical values.

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()

This doesn't work. So: I want to calculate my Groebner base, but with a treated as a constant. How do I do that ? This is just a dummy example, so I do not need help for the particular Groebner base of I, I want to know how to generally work with this kinds of constants and Groebner bases.