--- a/ProjectDefinition.st	Thu Jun 21 15:13:25 2018 +0200
+++ b/ProjectDefinition.st	Thu Jun 21 18:14:32 2018 +0200
@@ -1,5 +1,3 @@
-"{ Encoding: utf8 }"
-
 "
  COPYRIGHT (c) 2006 by eXept Software AG
 	      All Rights Reserved
@@ -1430,6 +1428,22 @@
     "Created: / 18-08-2006 / 16:21:01 / cg"
 !
 
+checkForBadReferences:prerequsiteList
+    "check, that there are no 'illegal' references to prerequisites.
+     If there is a bad prrequisite, ask the user whether to abort."
+
+    prerequsiteList keysDo:[:eachPackageSymbol|
+        |defClass|
+
+        defClass := self definitionClassForPackage:eachPackageSymbol.
+        (defClass isProjectDefinition and:[defClass isPluginDefinition]) ifTrue:[
+            (self confirm:('Bad cross-plugin requirement to: %1%<cr>%<cr>Proceed?' bindWith:eachPackageSymbol)) ifFalse:[
+                AbortOperationRequest raiseRequest.
+            ].
+        ].
+    ]
+!
+
 classNamesAndAttributes_codeFor:aSpecArray
     "generate method code returning all classes of the project from the given spec."
 
@@ -1922,26 +1936,29 @@
     |preRequisites|
 
     preRequisites := self searchForPreRequisites first.
-    preRequisites removeAllKeys:self excludedFromMandatoryPreRequisites ifAbsent:[].
-    preRequisites removeAllKeys:self excludedFromPreRequisites ifAbsent:[].
+    preRequisites 
+        removeAllKeys:self excludedFromMandatoryPreRequisites ifAbsent:[];
+        removeAllKeys:self excludedFromPreRequisites ifAbsent:[].
+
+    self checkForBadReferences:preRequisites.
 
     ^ String streamContents:[:s |
-	s nextPutLine:'mandatoryPreRequisites'.
-	s nextPutAll: '    "'; nextPutAll: (self class superclass lookupMethodFor: #mandatoryPreRequisites) comment; nextPutLine:'"'.
-	s nextPutLine:''.
-	s nextPutLine:'    ^ #('.
-	preRequisites keys asSortedCollection do:[:eachPackageID |
-	    |reason|
-
-	    s spaces:8.
-	    eachPackageID asSymbol storeOn:s.
-	    reason := preRequisites at:eachPackageID ifAbsent:[nil].
-	    reason notEmptyOrNil ifTrue:[
-		s nextPutAll:'    "'; nextPutAll:reason anElement; nextPut:$".
-	    ].
-	    s cr.
-	].
-	s nextPutLine:'    )'
+        s nextPutLine:'mandatoryPreRequisites'.
+        s nextPutAll: '    "'; nextPutAll: (self class superclass lookupMethodFor: #mandatoryPreRequisites) comment; nextPutLine:'"'.
+        s nextPutLine:''.
+        s nextPutLine:'    ^ #('.
+        preRequisites keys asSortedCollection do:[:eachPackageID |
+            |reason|
+
+            s spaces:8.
+            eachPackageID asSymbol storeOn:s.
+            reason := preRequisites at:eachPackageID ifAbsent:[nil].
+            reason notEmptyOrNil ifTrue:[
+                s nextPutAll:'    "'; nextPutAll:reason anElement; nextPut:$".
+            ].
+            s cr.
+        ].
+        s nextPutLine:'    )'
     ].
 
     "
@@ -2013,27 +2030,29 @@
     preRequisitesColl := self searchForPreRequisites.
     preRequisites := preRequisitesColl second.
     preRequisites
-	removeAllKeys:self excludedFromPreRequisites ifAbsent:[];
-	removeAllKeys:self excludedFromRequiredPreRequisites ifAbsent:[];
-	removeAllKeys:preRequisitesColl first keys ifAbsent:[].  "remove the mandatory prerequisites"
+        removeAllKeys:self excludedFromPreRequisites ifAbsent:[];
+        removeAllKeys:self excludedFromRequiredPreRequisites ifAbsent:[];
+        removeAllKeys:preRequisitesColl first keys ifAbsent:[].  "remove the mandatory prerequisites"
+
+    self checkForBadReferences:preRequisites.
 
     ^ String streamContents:[:s |
-	s nextPutLine:'referencedPreRequisites'.
-	s nextPutAll: '    "'; nextPutAll: (self class superclass lookupMethodFor: #referencedPreRequisites) comment; nextPutLine:'"'.
-	s nextPutLine:''.
-	s nextPutLine:'    ^ #('.
-	preRequisites keys asSortedCollection do:[:eachPackageID |
-	    |reason|
-
-	    s spaces:8.
-	    eachPackageID asSymbol storeOn:s.
-	    reason := preRequisites at:eachPackageID ifAbsent:[nil].
-	    reason notEmptyOrNil ifTrue:[
-		s nextPutAll:'    "'; nextPutAll:reason anElement; nextPut:$".
-	    ].
-	    s cr.
-	].
-	s nextPutLine:'    )'
+        s nextPutLine:'referencedPreRequisites'.
+        s nextPutAll: '    "'; nextPutAll: (self class superclass lookupMethodFor: #referencedPreRequisites) comment; nextPutLine:'"'.
+        s nextPutLine:''.
+        s nextPutLine:'    ^ #('.
+        preRequisites keys asSortedCollection do:[:eachPackageID |
+            |reason|
+
+            s spaces:8.
+            eachPackageID asSymbol storeOn:s.
+            reason := preRequisites at:eachPackageID ifAbsent:[nil].
+            reason notEmptyOrNil ifTrue:[
+                s nextPutAll:'    "'; nextPutAll:reason anElement; nextPut:$".
+            ].
+            s cr.
+        ].
+        s nextPutLine:'    )'
     ].
 
     "
@@ -2897,6 +2916,7 @@
     "Created: / 23-01-2007 / 19:08:27 / cg"
 ! !
 
+
 !ProjectDefinition class methodsFor:'description - project information'!
 
 applicationAdditionalIconFileNames
@@ -4995,6 +5015,7 @@
     ^ self subProjectMakeCallsUsing:'call vcmake %1 %2'.
 ! !
 
+
 !ProjectDefinition class methodsFor:'file templates'!
 
 autopackage_default_dot_apspec