code: public static Expression in(final Expression e1, final Expression e2) {
          return new RepeatDelimiter(IN, e1.isEnclosed() ? e1 : e1.enclose(), e2.isEnclosed() ? e2 : e2.enclose());
      }
docs: The IN expression.
  
  @param e1 The first expression.
  @param e2 The second expression.
  @return The expression.