diff --git a/lib/array.fs b/lib/array.fs
index e7fc5b7..12d32c4 100644
--- a/lib/array.fs
+++ b/lib/array.fs
@@ -5,7 +5,6 @@
 
 private{
 
-( === Allocation strategy === )
 defined? array-alloc-strategy [unless]
 
 \ Compute new capacity
@@ -21,8 +20,6 @@ struct
     int% field array>capa
 end-struct array%
 
-s" Index out of range" exception constant OUT-OF-RANGE export
-
 \ Allocate array with capacity
 : allocate-array ( n capa -- arr )
     array% %allocate throw
diff --git a/lib/core.fs b/lib/core.fs
index 297ee6d..f987629 100644
--- a/lib/core.fs
+++ b/lib/core.fs
@@ -16,8 +16,11 @@ s" Invalid argument" exception constant INVALID-ARGUMENT
     unless INVALID-ARGUMENT throw then
 ;
 
+( === Builtin Exceptions === )
+s" Index out of range" exception constant OUT-OF-RANGE export
+
 defined? roll [unless]
-    : roll ( w[n-1] ... w0 n -- w0 w[n-2] ... w0 w[n-1] )
+    : roll ( w[n-1] ... w0 n -- w[n-2] ... w0 w[n-1] )
         dup 0<= if drop else swap >r 1- recurse r> swap then
     ;
 [then]
diff --git a/lib/string.fs b/lib/string.fs
index 258ffd4..01a8cb3 100644
--- a/lib/string.fs
+++ b/lib/string.fs
@@ -1,23 +1,48 @@
 \ planckforth -
 \ Copyright (C) 2021 nineties
 
-( === String === )
+( === Heap Allocated String === )
 
 private{
 
-\ Heap-allocated string object
-\ p: null terminated string
-: make-string ( p -- str )
-    dup strlen 1 + allocate throw tuck strcpy
+: make-string ( c-addr -- str )
+    dup 0<> check-argument
+    dup strlen 1+ allocate throw
+    tuck strcpy
 ; export
 
-: release-string ( str -- ) free ; export
+: release-string ( str -- )
+    free
+; export
+
+: concat-string ( str1 str2 -- newstr )
+    dup 0<> check-argument
+    over 0<> check-argument
+    over strlen over strlen
+    ( str1 str2 n1 n2 )
+    over + 1+ allocate throw
+    ( str1 str2 n1 ptr )
+    3 pick over strcpy          \ copy str1 to ptr
+    ( str1 str2 n1 ptr)
+    tuck + 2 pick swap strcpy   \ copy str2 to ptr + n1
+    swap drop swap drop
+; export
 
 }private
 
 T{ s" AAAAA" make-string constant A -> }T
 T{ s" BBBBBBB" make-string constant B -> }T
+T{ s" " make-string constant C -> }T
+T{ A B concat-string constant D -> }T
 T{ A s" AAAAA" streq -> true }T
 T{ B s" BBBBBBB" streq -> true }T
+T{ C s" " streq -> true }T
+T{ D s" AAAAABBBBBBB" streq -> true }T
+T{ A strlen -> 5 }T
+T{ B strlen -> 7 }T
+T{ C strlen -> 0 }T
+T{ D strlen -> 12 }T
 T{ A release-string -> }T
 T{ B release-string -> }T
+T{ C release-string -> }T
+T{ D release-string -> }T