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.