diff --git a/compiler/guards.nim b/compiler/guards.nim
index 3df3bd1a8..ec5adb4da 100644
--- a/compiler/guards.nim
+++ b/compiler/guards.nim
@@ -246,6 +246,7 @@ proc canon*(n: PNode): PNode =
     result.sons[0] = opLen.newSymNode
   else: discard
 
+  result = skipConv(result)
   result = reassociation(result)
   # most important rule: (x-4) < a.len -->  x < a.len+4
   case result.getMagic
@@ -672,7 +673,9 @@ proc simpleSlice*(a, b: PNode): BiggestInt =
   else:
     result = -1
 
-proc ple(m: TModel; a, b: PNode): TImplication =  
+proc pleViaModel(model: TModel; aa, bb: PNode): TImplication
+
+proc ple(m: TModel; a, b: PNode): TImplication =
   template `<=?`(a,b): expr = ple(m,a,b) == impYes
   #   0 <= 3
   if a.isValue and b.isValue:
@@ -717,12 +720,68 @@ proc ple(m: TModel; a, b: PNode): TImplication =
     if a[1] <=? b or a[2] <=? b: return impYes
 
   # use the knowledge base:
-  return doesImply(m, opLe.buildCall(a, b))
+  return pleViaModel(m, a, b)
+  #return doesImply(m, opLe.buildCall(a, b))
+
+type TReplacements = seq[tuple[a,b: PNode]]
+
+proc replaceSubTree(n, x, by: PNode): PNode =
+  if sameTree(n, x):
+    result = by
+  elif hasSubTree(n, x):
+    result = shallowCopy(n)
+    for i in 0 .. safeLen(n)-1:
+      result.sons[i] = replaceSubTree(n.sons[i], x, by)
+  else:
+    result = n
+
+proc applyReplacements(n: PNode; rep: TReplacements): PNode =
+  result = n
+  for x in rep: result = result.replaceSubTree(x.a, x.b)
+
+proc pleViaModelRec(m: var TModel; a, b: PNode): TImplication =
+  # now check for inferrable facts: a <= b and b <= c  implies a <= c
+  for i in 0..m.high:
+    let fact = m[i]
+    if fact != nil and fact.getMagic in someLe:
+      # x <= y implies a <= b  if  a <= x and y <= b
+      let x = fact[1]
+      let y = fact[2]
+      # mark as used:
+      m[i] = nil
+      if ple(m, a, x) == impYes:
+        if ple(m, y, b) == impYes: return impYes
+        #if pleViaModelRec(m, y, b): return impYes
+
+proc pleViaModel(model: TModel; aa, bb: PNode): TImplication =
+  # compute replacements:
+  var replacements: TReplacements = @[]
+  for fact in model:
+    if fact != nil and fact.getMagic in someEq:
+      let a = fact[1]
+      let b = fact[2]
+      if a.kind == nkSym: replacements.add((a,b))
+      else: replacements.add((b,a))
+  var m: TModel
+  var a = aa
+  var b = bb
+  if replacements.len > 0:
+    m = @[]
+    # make the other facts consistent:
+    for fact in model:
+      if fact != nil and fact.getMagic notin someEq:
+        # XXX 'canon' should not be necessary here, but it is
+        m.add applyReplacements(fact, replacements).canon
+    a = applyReplacements(aa, replacements)
+    b = applyReplacements(bb, replacements)
+  else:
+    # we have to make a copy here, because the model will be modified:
+    m = model
+  result = pleViaModelRec(m, a, b)
 
 proc proveLe*(m: TModel; a, b: PNode): TImplication =
-  #echo "ROOT ", renderTree(a), " <=? ", b.rendertree
   let x = canon(opLe.buildCall(a, b))
-  #echo renderTree(res)
+  #echo "ROOT ", renderTree(x[1]), " <=? ", renderTree(x[2])
   result = ple(m, x[1], x[2])
   if result == impUnknown:
     # try an alternative:  a <= b  iff  not (b < a)  iff  not (b+1 <= a):
diff --git a/compiler/semparallel.nim b/compiler/semparallel.nim
index b13542038..678ffd2fb 100644
--- a/compiler/semparallel.nim
+++ b/compiler/semparallel.nim
@@ -59,7 +59,8 @@ type
   TDirection = enum
     ascending, descending
   MonotonicVar = object
-    v: PSym
+    v, alias: PSym        # to support the ordinary 'countup' iterator
+                          # we need to detect aliases
     lower, upper, stride: PNode
     dir: TDirection
     blacklisted: bool     # blacklisted variables that are not monotonic
@@ -83,7 +84,7 @@ proc initAnalysisCtx(): AnalysisCtx =
 
 proc lookupSlot(c: AnalysisCtx; s: PSym): int =
   for i in 0.. <c.locals.len:
-    if c.locals[i].v == s: return i
+    if c.locals[i].v == s or c.locals[i].alias == s: return i
   return -1
 
 proc getSlot(c: var AnalysisCtx; v: PSym): ptr MonotonicVar =
@@ -104,6 +105,11 @@ proc gatherArgs(c: var AnalysisCtx; n: PNode) =
         c.args.add root
     gatherArgs(c, n[i])
 
+proc isSingleAssignable(n: PNode): bool =
+  n.kind == nkSym and (let s = n.sym;
+    s.kind in {skTemp, skForVar, skLet} and
+          {sfAddrTaken, sfGlobal} * s.flags == {})
+
 proc isLocal(n: PNode): bool =
   n.kind == nkSym and (let s = n.sym;
     s.kind in {skResult, skTemp, skForVar, skVar, skLet} and
@@ -290,16 +296,16 @@ proc analyseCase(c: var AnalysisCtx; n: PNode) =
 proc analyseIf(c: var AnalysisCtx; n: PNode) =
   analyse(c, n.sons[0].sons[0])
   let oldFacts = c.guards.len
-  addFact(c.guards, n.sons[0].sons[0])
+  addFact(c.guards, canon(n.sons[0].sons[0]))
 
   analyse(c, n.sons[0].sons[1])
   for i in 1.. <n.len:
     let branch = n.sons[i]
     setLen(c.guards, oldFacts)
     for j in 0..i-1:
-      addFactNeg(c.guards, n.sons[j].sons[0])
+      addFactNeg(c.guards, canon(n.sons[j].sons[0]))
     if branch.len > 1:
-      addFact(c.guards, branch.sons[0])
+      addFact(c.guards, canon(branch.sons[0]))
     for i in 0 .. <branch.len:
       analyse(c, branch.sons[i])
   setLen(c.guards, oldFacts)
@@ -307,9 +313,12 @@ proc analyseIf(c: var AnalysisCtx; n: PNode) =
 proc analyse(c: var AnalysisCtx; n: PNode) =
   case n.kind
   of nkAsgn, nkFastAsgn:
-    # since we already ensure sfAddrTaken is not in s.flags, we only need to
-    # prevent direct assignments to the monotonic variable:
-    if n[0].isLocal:
+    if n[0].isSingleAssignable and n[1].isLocal:
+      let slot = c.getSlot(n[1].sym)
+      slot.alias = n[0].sym
+    elif n[0].isLocal:
+      # since we already ensure sfAddrTaken is not in s.flags, we only need to
+      # prevent direct assignments to the monotonic variable:
       let slot = c.getSlot(n[0].sym)
       slot.blackListed = true
     invalidateFacts(c.guards, n[0])
@@ -348,13 +357,13 @@ proc analyse(c: var AnalysisCtx; n: PNode) =
       # loop may never execute:
       let oldState = c.locals.len
       let oldFacts = c.guards.len
-      addFact(c.guards, n.sons[0])
+      addFact(c.guards, canon(n.sons[0]))
       analyse(c, n.sons[1])
       setLen(c.locals, oldState)
       setLen(c.guards, oldFacts)
       # we know after the loop the negation holds:
       if not hasSubnodeWith(n.sons[1], nkBreakStmt):
-        addFactNeg(c.guards, n.sons[0])
+        addFactNeg(c.guards, canon(n.sons[0]))
     dec c.inLoop
   of nkTypeSection, nkProcDef, nkConverterDef, nkMethodDef, nkIteratorDef,
       nkMacroDef, nkTemplateDef, nkConstSection, nkPragma:
@@ -429,6 +438,7 @@ proc liftParallel*(owner: PSym; n: PNode): PNode =
   # - detect monotonic local integer variables
   # - detect used slices
   # - detect used arguments
+  #echo "PAR ", renderTree(n)
   
   var a = initAnalysisCtx()
   let body = n.lastSon
diff --git a/tests/parallel/tforstmt.nim b/tests/parallel/tforstmt.nim
new file mode 100644
index 000000000..35f28759e
--- /dev/null
+++ b/tests/parallel/tforstmt.nim
@@ -0,0 +1,24 @@
+discard """
+  output: '''3
+4
+5
+6
+7'''
+  sortoutput: true
+"""
+
+import threadpool, math
+
+proc p(x: int) =
+  echo x
+
+proc testFor(a, b: int; foo: var openArray[int]) =
+  parallel:
+    for i in max(a, 0) .. min(b, foo.len-1):
+      spawn p(foo[i])
+
+var arr = [0, 1, 2, 3, 4, 5, 6, 7]
+
+testFor(3, 10, arr)
+
+