combi: fix broken k 0 edge cases for sequences.

* combi.c (rperm, comb, rcomb): In the default
case for generic sequences, check k, like
in the other cases and return the special
case result.

* tests/015/comb.tl: New tests.

1 files changed, 8 insertions, 0 deletions

diff --git a/combi.c b/combi.c
index 1eaade29..2b55ee08 100644
--- a/combi.c
+++ b/combi.c
@@ -471,6 +471,8 @@ val rperm(val seq, val k)
       return cons(string(L""), nil);
     return rperm_str(seq, k);
   default:
+    if (zerop(k))
+      return cons(make_like(nil, seq), nil);
     return rperm_seq(seq, k);
   }
 }
@@ -748,6 +750,10 @@ val comb(val seq, val k)
         return nil;
       return comb_hash(seq, k);
     }
+    if (k == zero)
+      return cons(make_like(nil, seq), nil);
+    if (gt(k, length(seq)))
+      return nil;
     return comb_seq(seq, k);
   }
 }
@@ -879,6 +885,8 @@ val rcomb(val seq, val k)
       return cons(string(L""), nil);
     return rcomb_str(seq, k);
   default:
+    if (k == zero)
+      return cons(make_like(nil, seq), nil);
     return rcomb_seq(seq, k);
   }
 }