x1 is any child of the first child of the first child of ... the first child of x2, where the graph geodesic through the ancestorship-directed family tree from x1 to x2 is of length (x3) + 1 (li; x3 must be a nonnegative integer or positive infinity), and where "first" is according to (partial) ordering rule x4 (default: chronological order of birth per successive generation within the given lineage; other restrictions on legitimacy etc. may be specified here as well).
8-ary mekso operator: the X1th nonnegative sum of X2 mutually-distinct perfect X3th-powers (i.e.: of integers) in X4 mutually truly-distinct ways, requiring exactly X5 terms to be negative in each sum (counting with(out^X6) multiplicity), requiring exactly X7 terms to be repeated between sums (counting with(out^X8) multiplicity), according to the usual ordering of the integers.