Create a concrete version of the substring
    :param start_idx : starting index of the substring
    :param end_idx : last index of the substring
    :param initial_string

    :return : a concrete version of the substring

def StrSubstr(start_idx, count, initial_string):
    """
    Create a concrete version of the substring
    :param start_idx : starting index of the substring
    :param end_idx : last index of the substring
    :param initial_string

    :return : a concrete version of the substring
    """
    new_value = initial_string.value[start_idx.value:start_idx.value + count.value]
    return StringV(new_value)