Trouble understanding Poisson Brackets

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I'm looking at page 94 here - I understand the definition of Poisson brackets at the top of the page (which uses summation convention) but I don't get why the calculations in (4.61) are true. I'm substituting in the generalised coordinate $q_i$ as $f$ and the generalised momentum $p_i$ as $g$, but I must be doing something wrong - do the subscripts in (4.61) not correspond to the subscripts in the definition?