ers.html#filter_helpers">filter helpers</A> mentioned above. The filter helpers produce filters which can be used for either input or output, but not both simultaneously. Such filters are called <A HREF="../concepts/dual_use_filter.html">Dual-Use Filters</A>.
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    A <A HREF="../concepts/dual_use_filter.html">Dual-Use Filter</A> can be produced by merging the implementations of an <A HREF="../concepts/input_filter.html">InputFilter</A> and an <A HREF="../concepts/output_filter.html">OutputFilter</A>