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mekso binary/unary operator: multinomial coefficient/binomial coefficient/choose

If unary, a list (n_1,...,n_r) must be supplied as input; the terms of the list may not be distinct; for appropriate values, output is ((n_1 +...+ n_r)!)/((n_1)! *...* (n_r)!). If binary, a single number n must be supplied for the first variable and the second variable must be a list (k_1,...,k_s); for appropriate values, the output is (n!)/((k_1 !) * ((n - k_1)!) *...* (k_s !) * ((n - k_s)!)); note that this is not exactly the typical definition, but it allows for easier immediate usage for the binomial case (where s=1).