There is a representable relation Sb1 such that for a formula , variable , and term , iff is substitutable for in .

The necessary definitions can be found in

http://cs.nyu.edu/courses/fall03/G22...c/lec12_h4.pdf (slide 6, 9)

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t is substitutable for x in if is an atomic formula, and so on (Enderton p 113).

How do I prove this problem using the definition of "t is substitutable for x in "?

Thanks.