code: def srcnode(self):
          
          srcdir_list = self.dir.srcdir_list()
          if srcdir_list:
              srcnode = srcdir_list[0].Entry(self.name)
              srcnode.must_be_same(self.__class__)
              return srcnode
          return self
docs: If this node is in a build path, return the node
          corresponding to its source file.  Otherwise, return
          ourself.