The Composite design pattern provides a solution:
Define separate Composite objects that compose the objects of a part-whole hierarchy into a tree structure.
Work through a common Component interface to treat Leaf and Composite objects uniformly.
Describing the Composite design in more detail is the theme of the following sections.
See Applicability section for all problems Composite can solve.

• The key concept in this pattern is to compose Leaf objects and Composite objects if any into higher-level Composite objects recursively (recursive composition). [BGI] The resulting structure is a tree structure that represents a part-whole hierarchy.
• Define separate Composite objects:
  
  • Define a class (Composite) that maintains a container of child Component objects (children) and forwards requests to these children (for each child in children: child.operation()).
  • For implementing child-related operations (like adding or removing child components to or from the container) see Implementation.
• "The key to the Composite pattern is an abstract class [Component] that represents both primitives and their containers." [GoF, p163] [BGI]
  • Clients can treat Leaf objects and entire Composite object structures uniformly (that is, clients do not know whether they are working with Leaf or Composite objects):
  • If the receiver of a request is a Leaf, the request is performed directly.
    If the receiver is a Composite, the request is performed on all Component objects downwards the hierarchy.
  • This greatly simplifies clients of complex hierarchies and makes them easier to implement, change, test, and reuse.