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232 II Seeing How It Works<br />

It’s also easy to see that t A t 1 is in the symmetry group of x þ tðxÞ. For the two squares<br />

in the rule<br />

t A t 1 defines the axis of reflection<br />

Still, in general, there’s more to check to ensure that there are right answers. More<br />

generally, rules with symmetric sums have inverses that aren’t the same—for example,<br />

this one<br />

When the squares are reflected, the left one is the left one and not the right one.<br />

There’s no flip.<br />

The precise details of applying rules and saying how they work are straightforward.<br />

It’s mostly using your eyes and saying what they see. A rule A fi B applies to a<br />

shape C in two easy stages.<br />

(1) Find a transformation t that makes the shape A part of C. This picks out some<br />

part of C that looks like A. In symbols it looks like this<br />

tðAÞ a C<br />

(2) Subtract the transformation of A from C, and then add the same transformation<br />

of the shape B. This replaces the part of C that’s like A with another part that looks like<br />

B. And once again, it’s nice to show it in symbols<br />

ðC<br />

tðAÞÞ þ tðBÞ

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