def prefix(prefix_, a_string):
    if prefix_[0] == a_string[0]:
        if len(prefix_) == 1: # if last item in prefix is also the same, return true
            return True
        else:
            if prefix(prefix_[1::], a_string[1::]):
                return True
            else:
                return prefix(prefix_, a_string[1::])
    else:
        return False

def is_substring(substring, a_string):
    try:
        if prefix(substring, a_string):
            return True
        else:
            return is_substring(substring, a_string[1::])
    except IndexError:
        return False

print(is_substring("", "gagnaskipan"))