FUNCTION: subposet - look for a given subposet within a poset

CALLING SEQUENCE:
subposet(q,p)
subposet(q,p,incl)
subposet(q,p,incl,img)

PARAMETERS:
q, p - posets
incl, img - optional arguments evaluating to a name

SYNOPSIS:

• subposet tests whether the poset q (its first argument) is a full subposet of the poset p (its second argument).

• if an inclusion of q into p as full subposet can be found, additional data about that inclusion may be obtained via the optional parameters incl and img. In particular:
  
  incl - a list containing data about the inclusion q --> p found. In fact incl[i] is the image in p of the point i of q under this inclusion.
  
  img - a vector marking the points of p belonging to the image of q by 1, those not belonging to the image by 0.

EXAMPLES:

> p:=[9, {[5, 6], [7, 2], [3, 2], [4, 1], [6], [8, 1, 9], [9, 3, 7], [1, 3]}];

> subposet(cr_K[2],p,'incl','img');

> print(img,incl);