Let P be the proc expression that you are supposed to write
for exercise 2 of assignment #4.  Then

    let f = proc (x) equal? (x, 17)
    in (P f)

returns 17, and

    let g = proc (x) less? (x, *(x, x))
    in (P f)

returns 2, and

    let h = proc (x) greater? (x, *(x, x))
    in (P f)

probably doesn't ever return anything.