code: protected String getUserDefinedFieldName(String field) {
          int index = field.indexOf('-');
          char letter = getUserDefinedFieldLetter();
  
          for (int i = 0; i < index; ++i) {
              if (field.charAt(i) == letter) {
                  return field.substring(index + 1);
              }
          }
  
          return null;
      }
docs: Returns the user defined field without its prefix.
  
  @param field the name of the user defined field
  @return the user defined field without the prefix, or null if the fields
  doesn't apply to this control file.
  @since 1.1