Informal Verification

• You give the definition of foo to Scheme and try some tests:

  Expression	Value
  (foo 0)	0
  (foo 1)	1
  (foo 2)	4
  (foo 3)	9
  (foo 4)	16

• Are you convinced that there is not somehow a mistake ("bug") in the procedure?

• Rigorous testing can give very good confirmation of the correctness of a procedure.

• But it is always possible that testing did not reveal all of the procedure's behavior: try (foo -3)