```python
from typing import List

def slug_word(word: str) -> str:
    """
    Lowercase a word and drop every character that is not a letter or a digit.
    """
    return ''.join(char.lower() for char in word if char.isalnum())

def slug_make(phrase: str) -> str:
    """
    Join the slugged words of a phrase with single hyphens, leaving out any word that slugs away to nothing.
    """
    slugged_words = [slug_word(word) for word in phrase.split()]
    return '-'.join(word for word in slugged_words if word)
```