  This paper analyses stable commutator length in groups Z^r * Z^s. We bound
scl from above in terms of the reduced wordlength (sharply in the limit) and
from below in terms of the answer to an associated subset-sum type problem.
Combining both estimates, we prove that, as n tends to infinity, words of
reduced length n generically have scl arbitrarily close to n/4 - 1.
  We then show that, unless P=NP, there is no polynomial time algorithm to
compute scl of efficiently encoded words in F2.
  All these results are obtained by exploiting the fundamental connection
between scl and the geometry of certain rational polyhedra. Their extremal rays
have been classified concisely and completely. However, we prove that a similar
classification for extremal points is impossible in a very strong sense.
