@@ -9,6 +9,9 @@ 
 proc raiseEmptyStack() =
   raise MinEmptyStackError(msg:"Insufficient items on the stack")
 
+proc raiseInvalid(msg: string) =
+  raise MinInvalidError(msg: msg)
+
 proc fullname*(scope: ref MinScope): string =
   result = scope.name
   if scope.parent.isNotNil:
@@ -40,6 +43,8 @@ proc setSymbol*(scope: ref MinScope, key: string, value: MinOperator): bool {.discardable.}=
   result = false
   # check if a symbol already exists in current scope
   if scope.isNotNil and scope.symbols.hasKey(key):
+    if scope.symbols[key].sealed:
+      raiseInvalid("Symbol '$1' is sealed." % key) 
     scope.symbols[key] = value
     result = true
   else:

@@ -85,6 +85,7 @@ MinOperatorKind* = enum
     minProcOp
     minValOp
   MinOperator* = object
+    sealed*: bool
     case kind*: MinOperatorKind
     of minProcOp:
       prc*: MinOperatorProc

@@ -219,19 +219,19 @@ scope.parent = i.scope
   return scope
 
 proc symbol*(scope: ref MinScope, sym: string, p: MinOperatorProc): ref MinScope =
-  scope.symbols[sym] = MinOperator(prc: p, kind: minProcOp)
+  scope.symbols[sym] = MinOperator(prc: p, kind: minProcOp, sealed: true)
   return scope
 
 proc symbol*(scope: ref MinScope, sym: string, v: MinValue): ref MinScope =
-  scope.symbols[sym] = MinOperator(val: v, kind: minValOp)
+  scope.symbols[sym] = MinOperator(val: v, kind: minValOp, sealed: true)
   return scope
 
 proc sigil*(scope: ref MinScope, sym: string, p: MinOperatorProc): ref MinScope =
-  scope.previous.sigils[sym] = MinOperator(prc: p, kind: minProcOp)
+  scope.previous.sigils[sym] = MinOperator(prc: p, kind: minProcOp, sealed: true)
   return scope
 
 proc sigil*(scope: ref MinScope, sym: string, v: MinValue): ref MinScope =
-  scope.previous.sigils[sym] = MinOperator(val: v, kind: minValOp)
+  scope.previous.sigils[sym] = MinOperator(val: v, kind: minValOp, sealed: true)
   return scope
 
 proc finalize*(scope: ref MinScope) =

@@ -65,7 +65,9 @@ i.debug "[define] " & symbol & " = " & $q1
       #let p = proc(i: In) =
       #  i.push q1.qVal
       #i.scope.symbols[symbol] = MinOperator(kind: minProcOp, prc: p)
-      i.scope.symbols[symbol] = MinOperator(kind: minValOp, val: q1)
+      if i.scope.symbols.hasKey(symbol) and i.scope.symbols[symbol].sealed:
+        raiseUndefined("Attempting to redefined sealed symbol '$1'" % symbol)
+      i.scope.symbols[symbol] = MinOperator(kind: minValOp, val: q1, sealed: false)
   
     .symbol("bind") do (i: In):
       var sym, val: MinValue

@@ -11,8 +11,11 @@ 
 proc sys_module*(i: In)=
   i.define("sys")
   
-    .symbol("pwd") do (i: In):
+    .symbol(".") do (i: In):
       i.push newVal(getCurrentDir())
+    
+    .symbol("..") do (i: In):
+      i.push newVal(getCurrentDir().parentDir)
     
     .symbol("cd") do (i: In):
       var f: MinValue

@@ -94,6 +94,8 @@ (first)
     ("Caught a " swap concat)
     ) try "Caught a MinEmptyStackError" ==) assert 
 
+  ("aaaa" :. . "aaaa" ==) assert ;It is possible to shadow sealed symbols in child scopes
+
   (
    (
     (("TestError" "Test Message") raise) 
@@ -130,6 +132,12 @@ 
   (((a 1)(b 2)(c 3)) values (1 2 3) ==) assert
 
   ("$1 - $2 - $3" (1 true "test") interpolate "1 - true - test" ==) assert
+
+  (((1 2 3)) :sym1 >sym1 stored-symbols "sym1" contains) assert
+
+  (<sym1 symbols "sym1" contains) assert
+
+  ('sym1 remove-symbol stored-symbols "sym1" contains false ==) assert
 
   report
   ; Tidy up

@@ -3,10 +3,10 @@ 'test import
 
 "sys" describe
 
-  ("systest" mkdir "." ls "./systest" contains) assert
+  ("systest" mkdir . ls . "/systest" concat  contains) assert
 
-  ("systest" cd pwd "systest" match) assert
-  ".." cd
+  ("systest" cd . "systest" match) assert
+  .. cd
 
   (&ls "\n" split "systest" contains) assert
 
@@ -28,9 +28,9 @@ ("test.txt" "test2.txt" cp "test2.txt" file?) assert
 
   ("test.txt" "test1.txt" mv "test1.txt" file?) assert
 
-  ("test2.txt" rm "test1.txt" rm pwd ls "test1.txt" contains :t1 pwd ls "test2" contains t1 and false ==) assert
+  ("test2.txt" rm "test1.txt" rm . ls "test1.txt" contains :t1 . ls "test2" contains t1 and false ==) assert
 
-  ("systest" rmdir pwd ls "systest" contains false ==) assert
+  ("systest" rmdir . ls "systest" contains false ==) assert
 
   report
   clear-stack