Exercise 4.15: Given a one-argument procedure
p and an object a, p is said to “halt” on a if
evaluating the expression (p a) returns a value (as opposed to
terminating with an error message or running forever).  Show that it is
impossible to write a procedure halts? that correctly determines whether
p halts on a for any procedure p and object a.  Use
the following reasoning: If you had such a procedure halts?, you could
implement the following program:


(define (run-forever)
  (run-forever))

(define (try p)
  (if (halts? p p)
      (run-forever)
      'halted))

Now consider evaluating the expression (try try) and show that any
possible outcome (either halting or running forever) violates the intended
behavior of halts?.227