Define the function in_dict(dict_arg, value) that returns a boolean True if value exists in dict_arg and False if the value is not in dict_arg.

Here's a sample implementation of the `in_dict` function in Python:

def in_dict(dict_arg, value):
    """
    Checks if a value exists in a dictionary.

    Args:
        dict_arg: A dictionary to search for the value.
        value: The value to search for in the dictionary.

    Returns:
        True if the value exists in the dictionary, False otherwise.
    """
    if value in dict_arg.values():
        return True
    else:
        return False

This function takes two arguments: `dict_arg`, which is the dictionary to search in, and `value`, which is the value to search for in the dictionary.

The function uses the `values()` method of the dictionary to get a list of all the values in the dictionary, and then checks if the `value` argument is in that list. If it is, the function returns `True`. Otherwise, it returns `False`.