In operator

The In operator (``in´´) tests if a given element is a member of a given set. This operation returns boolean, but the actual behaviour depends on the type of set. There is callback ``_in(element)´´ for classes, invoked with this expression. Negation of this (``!in´´) operation uses the same callback with a negation of the return value.

  in-expr := exprelement "in" exprset 
  in-expr := exprelement "!" "in" exprset 

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