code: public static int compare(XExpression e1, XExpression e2) {
  		if (e1 == e2) {
  			return 0;
  		}
  		if (e1 == null) {
  			return Integer.MIN_VALUE;
  		}
  		if (e2 == null) {
  			return Integer.MAX_VALUE;
  		}
  		return e1.toString().compareTo(e2.toString());
  	}
docs: Compare two Xtext expressions.
  
  @param e1 the first expression to compare.
  @param e2 the second expression to compare.
  @return A negative value if <code>e1</code> is
  lower than <code>e2</code>, a positive value if
  <code>e1</code> is greater than <code>e2</code>,
  otherwise <code>0</code>.