Fix slice() bug when index is off the end, doc fix

lists.scad

@@ -229,8 +229,9 @@ function select(list, start, end) =
 //   list = slice(list, s, e);
 // Description:
 //   Returns a slice of a list, from the first position `s` up to and including the last position `e`.
-//   The first item in the list is at index 0.  Negative indexes are counted back from the end.
+//   The first item in the list is at index 0.  Negative indexes are counted back from the end, with
-//   An index of -1 refers to the last list item.
+//   -1 referring to the last list item.  If `s` is after `e` then the empty list is returned.
+//   If an index is off the start/end of the list it will refer to the list start/end.  
 // Arguments:
 //   list = The list to get the slice of.
 //   start = The index of the first item to return.  Default: 0
@@ -243,6 +244,8 @@ function select(list, start, end) =
 //   d = slice([3,4,5,6,7,8,9], 5);      // Returns [8,9]
 //   e = slice([3,4,5,6,7,8,9], 2, -2);  // Returns [5,6,7,8]
 //   f = slice([3,4,5,6,7,8,9], 4, 3;    // Returns []
+//   g = slice([3,4,5], 1, 5;            // Returns [4,5]
+//   h = slice([3,4,5], 5, 7);           // Returns []
 function slice(list,start=0,end=-1) =
     assert(is_list(list))
     assert(is_int(start))
@@ -250,11 +253,10 @@ function slice(list,start=0,end=-1) =
     !list? [] :
     let(
         l = len(list),
-        start = constrain(start + (start<0? l : 0), 0, l-1),
+        start = start+(start<0 ? l : 0),
-        end = constrain(end + (end<0? l : 0), 0, l-1)
+        end = end + (end<0? l : 0)
     )
-    [if (end>=start) for (i=[start:1:end]) list[i]];
+    [if (start<=end && end>=0 && start<=l) for (i=[max(start,0):1:min(end,l-1)]) list[i]];
-
 
 // Function: last()
 // Usage:

skin.scad

@@ -139,7 +139,7 @@
 //   a vertex duplicating method on one side and a resampling method on the other side, then
 //   `refine` must be set so that the resulting number of vertices matches the number that is
 //   used for the resampled profiles.  The best way to avoid confusion is to ensure that the
-//   profiles connected by "direct" or "realign" all have the same number of points and at the
+//   profiles connected by "direct" or "reindex" all have the same number of points and at the
 //   transition, the refined number of points matches.
 //   .
 // Arguments:

tests/test_lists.scad

@@ -37,6 +37,14 @@ module test_slice() {
     assert(slice(l, 3, 3) == [6]);
     assert(slice(l, 4) == [7,8,9]);
     assert(slice(l, -2) == [8,9]);
+    assert(slice(l,-10,-8) == []);
+    assert(slice(l,10,12) == []);
+    assert(slice(l,12,10) == []);
+    assert(slice(l,4,12) == [7,8,9]);
+    assert(slice(l,-10,2) == [3,4,5]);
+    assert(slice(l,-10,-4) == [3,4,5,6]);
+    assert(slice(l,-1,1) == []);
+    assert(slice(l,5,4) == []);
 }
 test_slice();