<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def stem_word(self, word):
 """Stem a word to its common stem form."""

        if self.is_plural(word):
            return self.stem_plural_word(word)
        else:
            return self.stem_singular_word(word)