code: public <R> StreamEx<R> mapKeyValuePartial(
              BiFunction<? super K, ? super V, ? extends Optional<? extends R>> mapper
      ) {
          return mapPartial(toFunction(mapper));
      }
docs: Performs a mapping of the stream keys and values to a partial function
  removing the elements to which the function is not applicable.
  
  <p>
  If the mapping function returns {@link Optional#empty()}, the original
  entry will be removed from the resulting stream. The mapping function
  may not return null.
  
  <p>
  This is an <a href="package-summary.html#StreamOps">intermediate
  operation</a>.
  
  <p>
  The {@code mapKeyValuePartial()} operation has the effect of applying a
  one-to-zero-or-one transformation to the elements of the stream, and then
  flattening the resulting elements into a new stream.
  
  @param <R> The element type of the new stream
  @param mapper a <a
  href="package-summary.html#NonInterference">non-interfering </a>,
  <a href="package-summary.html#Statelessness">stateless</a>
  partial function to apply to original keys and values which returns a present optional
  if it's applicable, or an empty optional otherwise
  @return the new stream
  @since 0.6.8