Created a new Cocoa application using the standard template in Xcode.
Added Graph and Vertex classes.
Added import statements reflecting class hierarchy.
Added vertices property in Graph class. Lazily instantiating it.
Added adjacentVertices property in Vertex class. Lazily instantiating it.
Implemented addVertex: and removeVertex: methods in Vertex class.
Implemented addAdjacentVertex: and removeAdjacentVertex: method in Vertex class.