FUNCTION: readalg - interactive input of a bound poset algebra

CALLING SEQUENCE:
readalg()

PARAMETERS:
none

SYNOPSIS:

• readalg enters a dialog that allows to input a bound poset algebra interactively in the following way:
  
  First you are asked for the number of elements of the underlying poset.
  
  Next you are prompted for the upper neighbours of each element of the poset. For that purpose the elements are assumed to be the first positive integers from 1 up to the number of elements.
  Make sure, there are no trailing blanks at the end of the lists of upper neighbours for each point. If a point is maximal and thus has no upper neighbours, just type <return>.
  
  Finally you enter by pairs of points the definig zero relations. After having given all these relations, when the program waits for further input, terminate input with single <return>.

• As the data format for bound poset algebras without zero-relations is the same as for posets, readalg may also be used for interactive input of a poset.

EXAMPLES:

> a:=readalg();

Cardinality of the poset of the algebra ?
5
Input of the poset by upper neighbours :
1 : 2 3
2 : 4
3 : 4 5
4 :
5 :
Input of the relation pairs :
1 5