Your original goal seemed to be just an instance of

pred_setTheory.SUBSET_REFL

  |- !s. s SUBSET s

Are you actually trying to prove:

!A. IMAGE FST (A CROSS A) SUBSET A

?

If so, you could have:

Q.prove(
  `!A. IMAGE FST (A CROSS A) SUBSET A`,
  simp [pred_setTheory.SUBSET_DEF]
  \\ metis_tac []
  )

However, it may be better to prove a lemma for this, as it could be useful 
elsewhere. For example:

val image_fst_cross = Q.prove(
  `!A B. IMAGE FST (A CROSS B) = if B = EMPTY then EMPTY else A`,
  rw [pred_setTheory.EXTENSION, pred_setTheory.IN_CROSS,
      pred_setTheory.IN_IMAGE]
  \\ eq_tac
  \\ rw [] >- metis_tac []
  \\ fs []
  \\ qexists_tac `(x, x')`
  \\ simp []
  )

(There’s probably a nicer way of proving the above lemma.)

With this, the following works

Q.prove(
  `!A. IMAGE FST (A CROSS A) SUBSET A`, rw [image_fst_cross])

Anthony

> On 5 Feb 2016, at 07:19, Ramana Kumar <[email protected]> wrote:
> 
> What's the problem/what is going wrong?
> 
> On 5 February 2016 at 17:48, Nadeem Iqbal <[email protected]> wrote:
> Yes, I want the set of first components of the pairs. I replaced S with A, 
> and used IMAGE FST (A CROSS A), but the problem is not solved.
> 
> 
> 
> On Fri, Feb 5, 2016 at 8:52 AM, Ramana Kumar <[email protected]> 
> wrote:
> ``S CROSS S`` is a set of pairs. Although ``S`` is usually parsed as a 
> constant, so you might want to use a different letter, or use Parse.hide"S".
> 
> Do you want the set of first components of the pairs? That would be ``IMAGE 
> FST (S CROSS S)``, and should be equal to ``S``.
> 
> On 5 February 2016 at 14:42, Nadeem Iqbal <[email protected]> wrote:
> Hi,
> 
> Dear All,
> 
> I am working in HOL4. I want to extract the domain of the set of binary 
> relations.
> For example, when I tried this goal,
> 
> g `! S. (FST (S CROSS S)) SUBSET (FST (S CROSS S))`;
> 
> It could not run. Can any one guide me?
> 
> Regards,
> 
> Nadeem Iqbal
> HoD School of Computing and Information Sciences,
> Imperial College of Business Studies,
> Lahore, Pakistan.
> 
> 
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