from typing import Dict
from typing import List
from typing import Any


def merge_dicts_(
    a: Dict[str, List[Any]], b: Dict[str, List[Any]]
) -> Dict[str, List[Any]]:
    """
    >>> a = {}
    >>> merge_dicts_(a, {})
    {}
    >>> merge_dicts_(a, {'x': ['x1']})
    {'x': ['x1']}
    >>> merge_dicts_(a, {'x': ['x2']})
    {'x': ['x1', 'x2']}
    >>> merge_dicts_(a, {'x': ['x3'], 'y': ['y1']})
    {'x': ['x1', 'x2', 'x3'], 'y': ['y1']}
    """
    for k, v in b.items():
        if k not in a:
            a[k] = []
        a[k].extend(v)
    return a