Many systems are organized hierarchically. The CERNDOC documentation system is an example, as is the Unix file system, and the VMS/HELP system. A tree has the practical advantage of giving every node a unique name. However, it does not allow the system to model the real world. For example, in a hierarchical HELP system such as VMS/HELP, one often gets to a lead on a tree such as:
HELP COMPILER SOURCE_FORMAT PRAGMAS DEFAULTS
only to find a reference to another leaf: Please see
HELP COMPILER COMMAND OPTIONS DEFAULTS PRAGMAS
and it is necessary to leave the system and re-enter it. What was needed was a link from one node to another, because in this case the information was not naturally organized into a tree.
They mourn, but smile at length; and, smiling, mourn:
The tree will wither long before it fall:
The hull drives on, though mast and sail be torn;
The roof-tree sinks, but moulders on the hall
In massy hoariness; the ruined wall
Stands when its wind-worn battlements are gone;
The bars survive the captive they enthral;
The day drags through though storms keep out the sun;
And thus the heart will break, yet brokenly live on.
I have not loved the world, nor the world me;
I have not flattered its rank breath, nor bowed
To its idolatries a patient knee,—
Nor coined my cheek to smiles, nor cried aloud
In worship of an echo; in the crowd
They could not deem me one of such; I stood
Among them, but not of them; in a shroud
Of thoughts which were not their thoughts, and still could,
Had I not filed my mind, which thus itself subdued.
I have not loved the world, nor the world me,—
But let us part fair foes; I do believe,
Though I have found them not, that there may be
Words which are things,—hopes which will not deceive,
And virtues which are merciful, nor weave
Snares for the falling: I would also deem
O'er others' griefs that some sincerely grieve;
That two, or one, are almost what they seem,—
That goodness is no name, and happiness no dream.
There is a pleasure in the pathless woods,
There is a rapture on the lonely shore,
There is society where none intrudes,
By the deep Sea, and music in its roar:
I love not Man the less, but Nature more.