Base change

[mahrud]

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There should be a documentation page about base change. Also, this should probably work:

i1 : ZZ[a,b] ** QQ

o1 = QQ[a..b]

o1 : PolynomialRing

i2 : ZZ/101[a,b] ** QQ -- this one makes sense
stdio:2:13:(3): error: tensor product not implemented for these rings

i3 : ZZ[a,b] ** ZZ/101 -- this one doesn't
stdio:3:9:(3): error: no coefficient ring present

[mahrud]

Also this:

i1 : S = ZZ[x,y];

i2 : R = S ** S;

i3 : R ** QQ
stdio:3:3:(3): error: map: multiple variables would map to the same variable, by name