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).