def convert_tag(t):
    """Converts tags so that they can be used by WordNet:
    
    | Tag begins with | WordNet tag |
    |-----------------|-------------|
    | `N`             | `n`         |
    | `V`             | `v`         |
    | `J`             | `a`         |
    | `R`             | `r`         |
    | Otherwise       | `None`      |
    """        
    if t[0].lower() in {'n', 'v', 'r'}:
        return t[0].lower()
    elif t[0].lower() == 'j':
        return 'a'
    else:
        return None