summary_funs is a named list of expressions
representing commonly used expressions of summary functions for summarising
competition results with summarise_item(). Names of the elements will be
used as summary names. It is designed primarily to be used with long
format of competition results. To use them inside summarise_item()
use unquoting mechanism from rlang package.
Currently present functions:
min_score - min(score).
max_score - max(score).
mean_score - mean(score).
median_score - median(score).
sd_score - sd(score).
sum_score - sum(score).
num_games - length(unique(game)).
num_players - length(unique(player)).
Note that it is generally better to subset summary_funs using names
rather than indices because the order of elements might change in future
versions.