Index: library/nx/nx.tcl
===================================================================
diff -u -r65d13b7b37bf2ee5dabe0392eb1e7af3a0cc85fe -r83355de2812519cf29dc92cfdc2fc4892303f6ab
--- library/nx/nx.tcl	(.../nx.tcl)	(revision 65d13b7b37bf2ee5dabe0392eb1e7af3a0cc85fe)
+++ library/nx/nx.tcl	(.../nx.tcl)	(revision 83355de2812519cf29dc92cfdc2fc4892303f6ab)
@@ -2130,17 +2130,36 @@
       set args [list obj var [:namedParameterSpec {} value $options]]
       :public object method value=set $args {::nsf::var::set $obj $var $value}
     }
-    if {[:isMultivalued] && [:info lookup method value=add] eq "::nsf::classes::nx::VariableSlot::value=add"} {
-      set slotObj "slot=[::nsf::self]"
-      # lappend options_single slot=[::nsf::self]
-      if {$slotObj ni $options_single} {lappend options_single $slotObj}
-      set vspec [:namedParameterSpec {} value $options_single]
-      set addArgs [list obj prop $vspec {pos 0}]
-      :public object method value=add $addArgs {::nsf::next [list $obj $prop $value $pos]}
-      set delArgs [list obj prop -nocomplain:switch $vspec]
-      :public object method value=delete $delArgs {::nsf::next [list $obj $prop -nocomplain=$nocomplain $value]}
-    } else {
-      # TODO should we deactivate add/delete?
+
+    if {[:isMultivalued]} {
+      set baseMethods [lmap m {value=add value=delete} {
+        set mh [:info lookup method $m]
+        if {[string match "::nsf::classes::nx::VariableSlot::*" $mh]} {
+          set m
+        } else {
+          continue
+        }
+      }]
+
+      if {[llength $baseMethods]} {
+        set slotObj "slot=[::nsf::self]"
+        # lappend options_single slot=[::nsf::self]
+        if {$slotObj ni $options_single} {lappend options_single $slotObj}
+        set vspec [:namedParameterSpec {} value $options_single]
+        if {"value=add" in $baseMethods} {
+          set addArgs [list obj prop $vspec {pos 0}]
+          :public object method value=add $addArgs {::nsf::next [list $obj $prop $value $pos]}
+        }
+        
+        if {"value=delete" in $baseMethods} {
+          set delArgs [list obj prop -nocomplain:switch $vspec]
+          :public object method value=delete $delArgs \
+              {::nsf::next [list $obj $prop -nocomplain=$nocomplain $value]}
+        }
+      } else {
+        # TODO should we deactivate add/delete?
+      }
+  
     }
   }
 
@@ -2300,6 +2319,9 @@
   }
 
   ::nx::VariableSlot public method value=delete {obj prop -nocomplain:switch value} {
+    if {![:isMultivalued]} {
+      return -code error "property $prop of [set :domain] is not multivalued"
+    }
     set old [::nsf::var::get $obj $prop]
     set p [lsearch -glob $old $value]
     if {$p > -1} {
Index: tests/accessor.test
===================================================================
diff -u -r226d979a835578dbde618f73c37b22f65dafd238 -r83355de2812519cf29dc92cfdc2fc4892303f6ab
--- tests/accessor.test	(.../accessor.test)	(revision 226d979a835578dbde618f73c37b22f65dafd238)
+++ tests/accessor.test	(.../accessor.test)	(revision 83355de2812519cf29dc92cfdc2fc4892303f6ab)
@@ -6,80 +6,228 @@
 nx::test configure -count 1
 
 nx::test case setter-variants {
-    nx::Class create C {
-	:property {p1a 1}
-	:property {p1b 1} {
-	    :public object method value=set {obj prop value} {
-		nx::var::set $obj $prop [incr value]
-	    }
-	}
-	:property -accessor public {p2a 2}
-	:property -accessor public {p2b 2} {
-	    :public object method value=set {obj prop value} {
-		nx::var::set $obj $prop [incr value]
-	    }
-	}
+  nx::Class create C {
+    :property {p1a 1}
+    :property {p1b 1} {
+      :public object method value=set {obj prop value} {
+        nx::var::set $obj $prop [incr value]
+      }
+    }
+    :property -accessor public {p2a 2}
+    :property -accessor public {p2b 2} {
+      :public object method value=set {obj prop value} {
+        nx::var::set $obj $prop [incr value]
+      }
+    }
 
-	:property -incremental {p3a 3}
-	:property -incremental {p3b 3} {
-	    :public object method value=set {obj prop value} {
-		nx::var::set $obj $prop [incr value]
-	    }
-	}
+    :property -incremental {p3a 3}
+    :property -incremental {p3b 3} {
+      :public object method value=set {obj prop value} {
+        nx::var::set $obj $prop [incr value]
+      }
+    }
 
-	:create c1
+    :create c1
+  }
+
+  puts [C info method definition p1a]
+  ? {c1 cget -p1a} 1
+  ? {c1 configure -p1a 1} ""
+
+  puts [C info method definition p1b]
+  ? {c1 cget -p1b} 2
+  ? {c1 configure -p1b 3} ""
+  ? {c1 cget -p1b} 4
+
+  puts [C info method definition p2a]
+  ? {c1 cget -p2a} 2
+  ? {c1 p2a get} 2
+  ? {c1 configure -p2a 2} ""
+  ? {c1 p2a set 2} 2
+  ? {c1 p2a unset} ""
+  ? {c1 cget -p2a} {can't read "p2a": no such variable}
+  ? {c1 p2a unset} {can't unset "p2a": no such variable}
+  ? {c1 p2a unset -nocomplain} ""
+
+  puts [C info method definition p2b]
+  ? {c1 cget -p2b} 3
+  ? {c1 p2b get} 3
+  ? {c1 configure -p2b 2} ""
+  ? {c1 p2b set 2} 3
+  ? {c1 p2b unset} ""
+  ? {c1 cget -p2b} {can't read "p2b": no such variable}
+
+  puts [C info method definition p3a]
+
+  ? {c1 cget -p3a} 3
+  ? {c1 p3a get} 3
+  ? {c1 configure -p3a 3} ""
+  ? {c1 p3a get 3} {invalid argument '3', maybe too many arguments; should be "value=get ?-array? ?-notrace? /object/ /varName/"}
+  ? {c1 p3a set 3} 3
+  ? {c1 p3a unset} ""
+  ? {c1 cget -p3a} {can't read "p3a": no such variable}
+
+  puts [C info method definition p3b]
+
+  ? {c1 cget -p3b} 4
+  ? {c1 p3b get} 4
+  ? {c1 configure -p3b 4} ""
+  ? {c1 p3b get} 5
+  ? {c1 p3b set 4} 5
+  ? {c1 p3b get} 5
+  ? {c1 p3b set 4} 5
+  ? {c1 p3b get} 5
+
+  ? {c1 p3b unset} ""
+  ? {c1 cget -p3b} {can't read "p3b": no such variable}
+
+}
+
+nx::test case incremental-slot-override-wo-type {
+  nx::Class create C {
+    :property -accessor public c1a
+    :property -incremental c1b
+    :property -incremental c1c {
+      :public object method value=set {obj args} {
+        $obj eval [list lappend :trace [nx::current method]]
+        next
+      }
+      :public object method value=add {obj args} {
+        $obj eval [list lappend :trace [nx::current method]]
+        next
+      }
+      :public object method value=delete {obj args} {
+        $obj eval [list lappend :trace [nx::current method]]
+        next
+      }
     }
 
-    puts [C info method definition p1a]
-    ? {c1 cget -p1a} 1
-    ? {c1 configure -p1a 1} ""
+    :create c2
+  }
 
-    puts [C info method definition p1b]
-    ? {c1 cget -p1b} 2
-    ? {c1 configure -p1b 3} ""
-    ? {c1 cget -p1b} 4
+  # WITHOUT incremental being set: add + delete from base class are present, but CANNOT be called;
+  set slotObj1 [c2 info lookup variables c1a]
+  foreach m {value=set value=add value=delete value=get value=unset value=exists} {
+    ? [list $slotObj1 info lookup method $m] ::nsf::classes::nx::VariableSlot::$m
+  }
+  ? {c2 c1a add 0}    "property c1a of ::C is not multivalued"
+  ? {c2 c1a delete 0} "property c1a of ::C is not multivalued"
 
-    puts [C info method definition p2a]
-    ? {c1 cget -p2a} 2
-    ? {c1 p2a get} 2
-    ? {c1 configure -p2a 2} ""
-    ? {c1 p2a set 2} 2
-    ? {c1 p2a unset} ""
-    ? {c1 cget -p2a} {can't read "p2a": no such variable}
-    ? {c1 p2a unset} {can't unset "p2a": no such variable}
-    ? {c1 p2a unset -nocomplain} ""
+  # WITH incremental being set: add + delete from base class are present, and CAN be called;
+  set slotObj2 [c2 info lookup variables c1b]
+  foreach m {value=set value=add value=delete value=get value=unset value=exists} {
+    ? [list $slotObj2 info lookup method $m] ::nsf::classes::nx::VariableSlot::$m
+  }
+  ? {c2 c1b add 1}    "1"
+  ? {c2 c1b get}      "1"
+  ? {c2 c1b delete 1} ""
+  ? {c2 c1b get}      ""
+  ? {c2 c1b unset}      ""
 
-    puts [C info method definition p2b]
-    ? {c1 cget -p2b} 3
-    ? {c1 p2b get} 3
-    ? {c1 configure -p2b 2} ""
-    ? {c1 p2b set 2} 3
-    ? {c1 p2b unset} ""
-    ? {c1 cget -p2b} {can't read "p2b": no such variable}
+  # WITH incremental being set: add + delete from slot are present, override base methods, and CAN be called;
+  set slotObj3 [c2 info lookup variables c1c]
+  foreach m {value=set value=add value=delete} {
+    ? [list $slotObj3 info lookup method $m] ${slotObj3}::$m
+  }
+  foreach m {value=get value=unset value=exists} {
+    ? [list $slotObj3 info lookup method $m] ::nsf::classes::nx::VariableSlot::$m
+  }
+  ? {c2 info vars}    ""
+  ? {c2 c1c add 1}    "1"
+  ? {c2 c1c get}      "1"
+  ? {c2 c1c delete 1} ""
+  ? {c2 c1c get}      ""
+  ? {c2 c1c unset}      ""
+  ? {c2 info vars}    "trace"
+  ? {c2 eval {expr {"value=add" in ${:trace}}}} 1
+  ? {c2 eval {expr {"value=delete" in ${:trace}}}} 1
+  ? {c2 eval {expr {"value=set" in ${:trace}}}} 0
 
-    puts [C info method definition p3a]
+  
+}
 
-    ? {c1 cget -p3a} 3
-    ? {c1 p3a get} 3
-    ? {c1 configure -p3a 3} ""
-    ? {c1 p3a get 3} {invalid argument '3', maybe too many arguments; should be "value=get ?-array? ?-notrace? /object/ /varName/"}
-    ? {c1 p3a set 3} 3
-    ? {c1 p3a unset} ""
-    ? {c1 cget -p3a} {can't read "p3a": no such variable}
+nx::test case incremental-slot-override-with-type {
+  nx::Class create C {
+    :property -accessor public c1a:object
+    :property -incremental c1b:object
+    :property -incremental c1c:object {
+      :public object method value=set {obj prop value:object args} {
+        $obj eval [list lappend :trace [nx::current method]]
+        next
+      }
+      :public object method value=add {obj prop value:object args} {
+        $obj eval [list lappend :trace [nx::current method]]
+        next
+      }
+      :public object method value=delete {obj prop value:object args} {
+        $obj eval [list lappend :trace [nx::current method]]
+        next
+      }
+    }
 
-    puts [C info method definition p3b]
+    :create c3
+  }
+  
+  # WITHOUT incremental being set: set is overriden internally and becomes type-aware; add + delete
+  # from base class are present, but CANNOT be called;
+  set slotObj1 [c3 info lookup variables c1a]
+  foreach m {value=add value=delete value=get value=unset value=exists} {
+    ? [list $slotObj1 info lookup method $m] ::nsf::classes::nx::VariableSlot::$m
+  }
+  ? [list $slotObj1 info lookup method value=set] ${slotObj1}::value=set
 
-    ? {c1 cget -p3b} 4
-    ? {c1 p3b get} 4
-    ? {c1 configure -p3b 4} ""
-    ? {c1 p3b get} 5
-    ? {c1 p3b set 4} 5
-    ? {c1 p3b get} 5
-    ? {c1 p3b set 4} 5
-    ? {c1 p3b get} 5
+  ? {c3 c1a add 0}    "property c1a of ::C is not multivalued"
+  ? {c3 c1a delete 0} "property c1a of ::C is not multivalued"
+  ? {c3 c1a set 0}     {expected object but got "0" for parameter "value"}
+  ? {c3 c1a set [c3]}  [c3]
+  ? {c3 c1a unset}     ""
 
-    ? {c1 p3b unset} ""
-    ? {c1 cget -p3b} {can't read "p3b": no such variable}
+  # WITH incremental being set: set + add + delete are overriden INTERNALLY to make them type-aware;
+  set slotObj2 [c3 info lookup variables c1b]
+  foreach m {value=set value=add value=delete} {
+    ? [list $slotObj2 info lookup method $m] ${slotObj2}::$m
+  }
+  foreach m {value=get value=unset value=exists} {
+    ? [list $slotObj2 info lookup method $m] ::nsf::classes::nx::VariableSlot::$m
+  }
 
+  ? {c3 c1b add 1}    {expected object but got "1" for parameter "value"}
+  ? {c3 c1b delete 1} {expected object but got "1" for parameter "value"}
+  ? {c3 c1b set 1}    {invalid value in "1": expected object but got "1" for parameter "value"}
+  ? {c3 c1b add [c3]} [c3]
+  ? {c3 c1b delete [c3]} ""
+  ? {c3 c1b get} ""
+  ? {c3 c1b unset} ""
+
+  # WITH incremental being set: set + add + delete are overriden by
+  # the slot (Note: type-awareness must be taken care of explicitly, lost otherwise);
+  set slotObj3 [c3 info lookup variables c1c]
+  foreach m {value=set value=add value=delete} {
+    ? [list $slotObj3 info lookup method $m] ${slotObj3}::$m
+  }
+  foreach m {value=get value=unset value=exists} {
+    ? [list $slotObj3 info lookup method $m] ::nsf::classes::nx::VariableSlot::$m
+  }
+  ? {c3 info vars}       ""
+  ? {c3 c1c add 1}       {expected object but got "1" for parameter "value"}
+  ? {c3 c1c delete 1}    {expected object but got "1" for parameter "value"}
+  ? {c3 c1c set 1}       {expected object but got "1" for parameter "value"}
+  ? {c3 c1c add [c3]}    [c3]
+  ? {c3 c1c delete [c3]} ""
+  ? {c3 c1c get}         ""
+  ? {c3 c1c unset}       ""
+  ? {c3 info vars}       "trace"
+  
+  ? {c3 eval {expr {"value=add" in ${:trace}}}}    1
+  ? {c3 eval {expr {"value=delete" in ${:trace}}}} 1
+  ? {c3 eval {expr {"value=set" in ${:trace}}}}    0
+  ? {c3 c1c set [c3]}                              [c3]
+  ? {c3 eval {expr {"value=set" in ${:trace}}}}    1
 }
 
+
+# Local variables:
+#    mode: tcl
+#    tcl-indent-level: 2
+#    indent-tabs-mode: nil
+# End:
Index: tests/properties.test
===================================================================
diff -u -r13ac2740ff99a0438d547250401ac9ed981c58bf -r83355de2812519cf29dc92cfdc2fc4892303f6ab
--- tests/properties.test	(.../properties.test)	(revision 13ac2740ff99a0438d547250401ac9ed981c58bf)
+++ tests/properties.test	(.../properties.test)	(revision 83355de2812519cf29dc92cfdc2fc4892303f6ab)
@@ -163,9 +163,11 @@
 
   set unknowns "valid are: {assign definition destroy get getParameterSpec getPropertyDefinitionOptions onError parameter reconfigure setCheckedInstVar}"
   ? {c1 b add x} {property b of ::C is not multivalued}
+  ? {c1 b delete x} {property b of ::C is not multivalued}
   #? {c1 b add x} "method 'add' unknown for slot ::C::slot::b; $unknowns"
   ? {c1 c add x} {::c1: unable to dispatch method 'c'}
   ? {c1 eval {:c add x}} {property c of ::C is not multivalued}
+  ? {c1 eval {:c delete x}} {property c of ::C is not multivalued}
   #? {c1 eval {:c add x}} "method 'add' unknown for slot ::C::slot::c; $unknowns"
   ? {c1 d add x} {::c1: unable to dispatch method 'd'}
   ? {c1 eval {:d add x}} {::c1: unable to dispatch method 'd'}
@@ -178,9 +180,11 @@
 
   ? {c1 va add x} {::c1: unable to dispatch method 'va'}
   ? {c1 vb add x} {property vb of ::C is not multivalued}
+  ? {c1 vb delete x} {property vb of ::C is not multivalued}
   #? {c1 vb add x} "method 'add' unknown for slot ::C::slot::vb; $unknowns"
   ? {c1 vc add x} {::c1: unable to dispatch method 'vc'}
   ? {c1 eval {:vc add x}} {property vc of ::C is not multivalued}
+  ? {c1 eval {:vc delete x}} {property vc of ::C is not multivalued}
   #? {c1 eval {:vc add x}} "method 'add' unknown for slot ::C::slot::vc; $unknowns"
   ? {c1 vd add x} {::c1: unable to dispatch method 'vd'}
   ? {c1 eval {:vd add x}} {::c1: unable to dispatch method 'vd'}