  The recursion tree resulting from Karatsuba's formula is built here by using
an interleaved splitting scheme rather than the traditional left/right one.
This allows an easier access to the nodes of the tree and $2n-1$ of them are
initially flattened all at once into a single recursive formula. The whole tree
is then flattened further into a convolution formula involving less elementary
multiplications than the usual Cauchy product. Unlike the traditional splitting
scheme, the interleaved approach may also be applied to infinite power series,
and corresponding formulas are also given.
