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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _starting_consonants_only(self, letters: list) -> list: """Return a list of starting consonant positions."""

        for idx, letter in enumerate(letters):
            if not self._contains_vowels(letter) and self._contains_consonants(letter):
                return [idx]
            if self._contains_vowels(letter):
                return []
            if self._contains_vowels(letter) and self._contains_consonants(letter):
                return []
        return []