code: public static Collection<String> importProperty(MutableExtension extension, String propertySuffix,
          Collection<String> def)
      {
          Object obj = importProperty(extension, propertySuffix);
  
          if (obj == null) {
              return def;
          } else if (obj instanceof Collection) {
              return (Collection) obj;
          } else if (obj instanceof String[]) {
              return Arrays.asList((String[]) obj);
          } else {
              return importPropertyStringList(obj.toString(), true);
          }
      }
docs: Delete and return the value of the passed special property as a Collection of Strings.
  
  @param extension the extension from which to extract custom property
  @param propertySuffix the property suffix
  @param def the default value
  @return the value or {@code def} if none was found
  @since 8.3M1