1. almost regardless of f,  f^K  for sufficiently large K is likely to
 be fairly dense.
2. You could encode f more tightly as a univariate polynomial by using
different N, depending on the degree
 of x,y,z, etc.
3. (most plausible) you could use a recursive representation for f,
which could, I think, also solve your 100-variable problem by suitable
ordering of variables.
4. my experience is that you can multiply polynomials that are not too
large or too sparse lots of ways that are faster than the heapmul by
Monagan and Pearce, but your implementation experience may differ.
5. Maxima's CRE form is already in Sage, and does (3).
 RJF

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