If I try this, it fails:
R.<x,y> = PolynomialRing(QQ, 'x, y')
R.extend_variables('z')
AttributeError: 'sage.rings.polynomial.multi_polynomial_libsingular.MPolynomialRing_libsingular' object has no attribute 'extend_variables'(But a univariate polynomial ring PolynomialRing(QQ, 'x') has an extend_variables function.)
How do I take a polynomial ring R and make a new ring R′ with more variables, such that R⊂R′?
