Rename 'set_membership' to 'elem'

lisa-front/src/main/scala/lisa/front/parser/FrontResolver.scala

@@ -96,7 +96,7 @@ private[parser] object FrontResolver {
     case operator: ParsedBinaryOperator =>
       val label: Either[PredicateLabel[?], ConnectorLabel[?]] = operator match {
         case _: ParsedEqual => Left(equality)
-        case _: ParsedMembership => Left(ConstantPredicateLabel[2]("set_membership"))
+        case _: ParsedMembership => Left(ConstantPredicateLabel[2]("elem"))
         case _: ParsedSubset => Left(ConstantPredicateLabel[2]("subset_of"))
         case _: ParsedSameCardinality => Left(ConstantPredicateLabel[2]("same_cardinality"))
         case _: ParsedAnd => Right(and)

lisa-theories/src/main/scala/lisa/settheory/SetTheoryDefinitions.scala

@@ -16,7 +16,7 @@ private[settheory] trait SetTheoryDefinitions {
   /**
    * The symbol for the set membership predicate.
    */
-  final val in = ConstantPredicateLabel("set_membership", 2)
+  final val in = ConstantPredicateLabel("elem", 2)
 
   /**
    * The symbol for the subset predicate.