*e
from
AlloyのRelation
反射推移閉包
https://alloytools.org/tutorials/online/sidenote-rtc.html
https://alloy.readthedocs.io/en/latest/language/sets-and-relations.html#and
^e
の自身も含む版
*r
は、
iden
+
^r
と同じ
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