x1 is path-connected to x2 via the (non-gendered/absolute) first-born progeny/primogeniture relation; x1 is the first-born offspring of the first-born offspring of ... the first-born offspring of x2, connected by a graph-path passing through intermediate relations/steps (successive first-born offspring) x3 (ce'o; ordered list), "first" being according to ordering rule x4 (default, as used in this definition: chronological order of birth; restrictions on legitimacy etc. may also be included here).
Despite the definition saying "first-born" repeatedly, other orderings may be specified by x4, in which case "firstness" is according to the specified rules (and applies to x1 and the members of x3 (but not necessarily x2) each in turn for their generation and lineage (in other words: each of child will be the 'first' child of the previous member, until but not necessarily including x2, where "firstness" is as defined; importantly, the "firstness" relation will not be compared across generations (it renews with each generation) or across lineages - it is a partial ordering). This word's usage will typically be restricted to biological relations (otherwise, what is the point/what does the word mean? Additionally, "panzi" is seemingly restrictive in this regard). Append with seltau "sarcu" for qualified/conditional primogenture; append with tertau "cerda" in order to reference primogeniturial inheritance.