from typing import List


def find_positions(arr: List[str],
                   mask: List[bool]) -> List[int]:
    """Set positions and tokens.
    
    Args:
        tokens: List of tokens and untokens.
        mask: Mask for tokens. 
        
    Returns:
        List of positions of tokens.
        
    """
    pos = []
    for i, (token, istoken) in enumerate(zip(arr, mask)):
        if istoken:
            pos.append(i)
    return pos