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)
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 "?