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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def apply_subfield_projection(field, value, deep=False):
 """Apply projection from request context. The passed dictionary may be mutated. :param field: An instance of `Field` or `Serializer` :type field: `Field` or `Serializer` :param value: Dictionary to apply the projection to :type value: dict :param deep: Also process all deep projections :type deep: bool """

    # Discover the root manually. We cannot use either `self.root` or `self.context`
    # due to a bug with incorrect caching (see DRF issue #5087).
    prefix = []
    root = field
    while root.parent is not None:
        # Skip anonymous serializers (e.g., intermediate ListSerializers).
        if root.field_name:
            prefix.append(root.field_name)
        root = root.parent
    prefix = prefix[::-1]

    context = getattr(root, '_context', {})

    # If there is no request, we cannot perform filtering.
    request = context.get('request')
    if request is None:
        return value

    filtered = set(request.query_params.get('fields', '').split(FIELD_SEPARATOR))
    filtered.discard('')
    if not filtered:
        # If there are no fields specified in the filter, return all fields.
        return value

    # Extract projection for current and deeper levels.
    current_level = len(prefix)
    current_projection = []
    for item in filtered:
        item = item.split(FIELD_DEREFERENCE)
        if len(item) <= current_level:
            continue

        if item[:current_level] == prefix:
            if deep:
                current_projection.append(item[current_level:])
            else:
                current_projection.append([item[current_level]])

    if deep and not current_projection:
        # For deep projections, an empty projection means that all fields should
        # be returned without any projection.
        return value

    # Apply projection.
    return apply_projection(current_projection, value)