[see also: describe, account, reason, justify]
This explains why we chose 9 rather than, say, 1 for the second coordinate.
In Chapter 5, we shall explain what it means for a subset $V$ of $A$ to be determining for the centre of $X$.
To obtain the required map one must modify the method as explained in [9].
Any other unexplained notation is as found in Fox (1995).
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