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The sheet of assent itself is of the nature of a graph; for it represents the idea of the universe which graphist and interpreter mutually agree to be true at the outset of their conversation.
The sheet together with all that is at any time scribed on it is likewise a graph.

I now hasten to Convention No. 2 which is of all the conventions the one which is most arbitary and most characteristic of this special system of expression.

The following is a Supplement to Convention No 2.
It is best to conceive that the blackboard is only a part of that sheet, namely, that part to which attention is directed at the moment.
The rest of the sheet of assent is an imaginary extension of the board.
Thus, when a graph is erased from the board, assent to it is no necessarily withdrawn.
It is only moved away to a distant part of the sheet.