The current signature of Matrix.dot for two frames is:

df1:Frame<'R, 'C> * df2:Frame<'C, 'R> -> Frame<'R,'R>

But why do the 'R types have to match? This seems to be more flexible and make more sense:

df1:Frame<'R0, 'C> * df2:Frame<'C, 'R1> -> Frame<'R0,'R1>