To give an alternative definition of ordered pairs, choose two different sets *square* and *triangle* (these appear as actual squares and triangles in the text). For example, *square* = { }, *triangle* = {{ }}. Define
(a,b) = {{a,*square*}, {b,*triangle*}}.
State and prove an analogue of Theorem 1.2 -- (a,b) = (a',b') if and only if a = a' and b = b' -- for this notion of ordered pairs.
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I'm utterly lost...