code: public static Boolean negate(final Boolean bool) {
          if (bool == null) {
              return null;
          }
          return bool.booleanValue() ? Boolean.FALSE : Boolean.TRUE;
      }
docs: <p>Negates the specified boolean.</p>
  
  <p>If {@code null} is passed in, {@code null} will be returned.</p>
  
  <p>NOTE: This returns null and will throw a NullPointerException if unboxed to a boolean. </p>
  
  <pre>
  BooleanUtils.negate(Boolean.TRUE)  = Boolean.FALSE;
  BooleanUtils.negate(Boolean.FALSE) = Boolean.TRUE;
  BooleanUtils.negate(null)          = null;
  </pre>
  
  @param bool  the Boolean to negate, may be null
  @return the negated Boolean, or {@code null} if {@code null} input