New Union implementation. Output format change to one existing test, …

…new test.

Improve test file to get 100% coverage of new code. Fix some bugs. Document change.

Add a sweep phase to deal with the matching stride case faster.

dev/Updates/union

@@ -0,0 +1,24 @@
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+% Format 'yyyy/mm/dd'
+!! Date
+2016/01/11
+!! Changed by
+SL
+! Reported by
+Nick Loughlin
+!! Type of Change
+Fix: wrong result
+
+!! Description
+Under some circumstances the union of a combination of lists of small integers
+and ranges could be computed incorrectly. This change fixes this and also
+avoids expanding ranges unnecessarily in some cases.
+
+! Test Code
+# included in union.tst
+
+! Prefetch
+
+!! Changeset
+
+!! End

lib/coll.gd

@@ -3092,7 +3092,10 @@ DeclareOperation( "Intersection2",
 ##  [ 1, 2, 3, 4 ]
 ##  gap> Union( [ ] );
 ##  [  ]
-##  ]]></Example>
+##  ]]></Example><P/>
+##  When computing the Union of lists or sets of small integers and ranges, 
+##  every attempt is made to return the result as a range and to avoid expanding
+##  ranges provided as input.
 ##  </Description>
 ##  </ManSection>
 ##  <#/GAPDoc>