code: @Nonnull
    @ReturnsMutableObject ("passed parameter")
    static byte [] _encode3to4 (@Nonnull final byte [] b4,
                                @Nonnull final byte [] threeBytes,
                                @Nonnegative final int numSigBytes,
                                final int options)
    {
      _encode3to4 (threeBytes, 0, numSigBytes, b4, 0, options);
      return b4;
    }
docs: Encodes up to the first three bytes of array <var>threeBytes</var> and
  returns a four-byte array in Base64 notation. The actual number of
  significant bytes in your array is given by <var>numSigBytes</var>. The
  array <var>threeBytes</var> needs only be as big as <var>numSigBytes</var>.
  Code can reuse a byte array by passing a four-byte array as <var>b4</var>.
  
  @param b4
  A reusable byte array to reduce array instantiation
  @param threeBytes
  the array to convert
  @param numSigBytes
  the number of significant bytes in your array
  @return four byte array in Base64 notation.
  @since 1.5.1