## Two typographical errors

- In both the published version of the paper and the preprint version here,
in the section "Sampling the liabilities at the tips", at the end of the
section there are quantities such as p
_{c}+2. These should of course
be p_{c+2}, and so on. The +1, +2, etc. are supposed to be in the
subscripts.
- Equation (2) is printed incorrectly. The numerator of the fraction should
have two more parentheses and be

(x_{ki} - x'_{ki}) (x_{kj} - x'_{kj})

This is printed correctly in the preprint version, but is incorrect in the
printed version.