EDB — 239

[239]Prerequisites:[01R]↺↻,[(3.171)]↺↻,[24V]↺↻.Let $x,y$ be elements (generic, not necessarily natural numbers), such that

prove that

where the above two are mutually exclusive, and (in the hypothesis ?? above) the second one holds if and only if $x\in y$; summarizing

Note the analogy with [22H]↺↻.