title: remove pairs of equal elements from two lists in sympy
body: <p>I am quite new to Sympy (running with Python 3).<br />
  Given two lists of integers, in general not of equal lengths and whose elements might repeat, I need to eliminate all pairs of equal elements between the two lists.
  For example, given<br />
  List_a=[1, 2, 3, 4, 4], List_b=[0, 2, 2, 4]<br />
  I shall eliminate the pairs (2,2) and (4,4) and output<br />
  List_ar=[1,3,4], List_br=[0,2]<br />
  If the two lists are equal I shall get two empty lists as output.<br />
  I have been trying to compare the lists element by element ( &quot;for&quot; and &quot;while&quot; loops) and when found equal to delete the pair from both lists. Upon that to repeat the procedure on the reduced lists till no deletion is made.
  But lacking of a &quot;goto&quot; control I do not know how to handle the variable length lists.<br />
  Thanks for helping.</p>