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# lex_le(+List1, +List2)

List1 is lexicographically less or equal to List2

List1
  List of integers or domain variables
List2
  List of integers or domain variables

## Description

Imposes a lexicographic ordering between the two lists. I.e. either is the first element of List1 strictly smaller than the first element of List2, or the first elements are equal and the lexicographic order holds between the two list tails. A non-existing element (i.e. when the end of list is reached)is strictly smaller than any existing element.

This is currently a prototype -- the constraint has not been tested very extensively and little effort has been spent to optimise performance. We welcome any feedback on using this constraint.

This constraint is known as lex_lesseq in the global constraint catalog. The implementation here maintains generalised arc consistency and uses the algorithm described in: Z. Kiziltan, 'Symmetry Breaking Ordering Constraints, Ph.D thesis, Uppsala University, 2004.