code: def StrReplace(initial_string, pattern_to_be_replaced, replacement_pattern):
      
      new_value = initial_string.value.replace(pattern_to_be_replaced.value,
                                               replacement_pattern.value,
                                               1)
      return StringV(new_value)
docs: Create a concrete version of the replaced string
      (replace ONLY th efirst occurrence of the pattern)
  
      :param initial_string: string in which the pattern needs to be replaced
      :param pattern_to_be_replaced: substring that has to be replaced inside initial_string
      :param replacement_poattern: pattern that has to be inserted in initial_string t replace
                                   pattern_to_be_replaced
      :return: a concrete representation of the replaced string