Implement PositionsBound.

Currently there is no library method to return the set of all bound
entries of a list. This commit implements this functionality in an
efficient manner.

doc/ref/lists.xml

@@ -1410,6 +1410,7 @@ The latter can be done also using <Ref Func="ListWithIdenticalEntries"/>.
 <#Include Label="PositionProperty">
 <#Include Label="PositionsProperty">
 <#Include Label="PositionBound">
+<#Include Label="PositionsBound">
 <#Include Label="PositionNot">
 <#Include Label="PositionNonZero">
 <#Include Label="PositionSublist">

lib/list.gd

@@ -1002,6 +1002,34 @@ DeclareOperation( "PositionsProperty", [ IsList, IsFunction ] );
 DeclareOperation( "PositionBound", [ IsList ] );
 
 
+#############################################################################
+##
+#O  PositionsBound( <list> ) . . . . . . . . . positions of all bound entries
+##
+##  <#GAPDoc Label="PositionsBound">
+##  <ManSection>
+##  <Oper Name="PositionsBound" Arg='list'/>
+##
+##  <Description>
+##  returns the set of all indices for which an element is bound in the list
+##  <A>list</A>.
+##  <P/>
+##  <Example><![CDATA[
+##  gap> PositionsBound([1,2,3]);
+##  [ 1, 2, 3 ]
+##  gap> PositionsBound([,1,,3]);
+##  [ 2, 4 ]
+##  gap> PositionsBound([]);
+##  []
+##  ]]></Example>
+##  </Description>
+##  </ManSection>
+##  <#/GAPDoc>
+##
+DeclareOperation( "PositionsBound", [ IsList ] );
+
+
+
 #############################################################################
 ##
 #O  PositionSublist( <list>, <sub>[, <from>] )

lib/list.gi

@@ -1692,6 +1692,27 @@ InstallMethod( PositionBound,
     end );
 
 
+#############################################################################
+##
+#M  PositionsBound( <list> ) . . . . . . . . . positions of all bound entries
+##
+InstallMethod( PositionsBound,
+    "for a list",
+    [ IsList ],
+    function( list )
+    local i, bound, pos;
+    bound := [];
+    pos := 1;
+    for i in [ 1 .. Length( list ) ] do
+        if IsBound( list[i] )  then
+            bound[pos] := i;
+            pos := pos + 1;
+        fi;
+    od;
+    return bound;
+    end );
+
+
 #############################################################################
 ##
 #M  PositionSublist( <list>,<sub>[,<ind>] )