x1 is any child of the first child of the first child of ... the first child of x2, with intermediate steps x3 (complete ordered list, "ce'o"; should include the relevant parent of x1), 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).
The children of x2 are valid candidates for x1 due to trivial contraction of the relation. As an example: if Queen Elizabeth II of the UK/Commonwealth is x2 and chronological order of birth for legitimate children (gender-neutral primogeniture on legitimate descent) is used as x4, then the set of possible x1s includes (and, as of 9 May A.D. 2019, is equal to the set of) all of the children of Elizabeth II, the legitimate children of her son Charles (William, Harry), and all of William's children (George, Charlotte, Louis); notice that the descendants of Elizabeth II not via Charles are excluded, as are any descendants not by William (including Harry's children); the next generation of possible x1s will be (supposing that they come to exist) the children of George (the first-born legitimate son of William first-born legitimate son of Charles first-born legitimate son of Queen Elizabeth II) but not the descendants of William not by George; the fact that all of the intermediate steps are male is mere coincidence; this selbri in this example is a generalization of how the title "prince" is automatically bestowed in this line (excluding by marriage), at present, except for those who are granted princehood by virtue of being the children of the sons of the sovereign. (Technically, in at least some contexts, x3 would have to be deleted or made variable in order to specify all such princes because any given sequence of intermediate steps, even if elliptical, will essentially uniquely specify the parent of x1). See: "4 (default, as used in this definition: chronological order of birth; restrictions on legitimacy etc. may also be included here).">pavmompazy'utka".