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covers    --    a poset description from the list of the strict relations


Syntax 
covers(relP: LIST): LIST  of LIST

Description
This function print a poset description from the list relP of the strict relations in a poset.

Example 
// POSET:
//         3   4
//          \ /
//           2
//           |
//           1
/**/ relP := [[1,2],  [2,3],  [2,4]];
/**/ covers(relP);
[[],  [1],  [1,  2],  [1,  2]]

/**/ indent(covers(relP));
[
  [],      // elems < 1
  [1],     // elems < 2
  [1,  2], // elems < 3
  [1,  2]  // elems < 4
]