isup: Various changes and fixes to the MessageStructure

* Use parameterName instead of name to avoid funny effects.
* Handle tag only classes as fixed field with a zero lengthLength
* Make the structure classes return an instance of itself. This
  is a bit weird and will need more thinking.

MessageStructure.st

@@ -103,7 +103,7 @@ Object subclass: MSGField [
     <category: 'osmo-networking'>
     <comment: 'The description of an Information Element'>
 
-    MSGField class >> name [
+    MSGField class >> parameterName [
         <category: 'accessing'>
 	^ self subclassResponsibility
     ]
@@ -113,6 +113,13 @@ Object subclass: MSGField [
 	^ self subclassResponsibility
     ]
 
+    MSGField class >> lengthLength [
+        "The length of the length field. The default is to assume a length of
+        one octet and in the units of octets"
+        <category: 'accessing'>
+        ^ 1
+    ]
+
     MSGField class >> octalLength [
         <category: 'accessing'>
 	^ self subclassResponsibility

isup/create_structs.st

@@ -167,7 +167,7 @@ Object subclass: StructCreator [
     handleFixedLength: aDef [
         "Some fields have conflicting types... E.g. Range and Status
          appears sometimes only as range... without the status."
-        | len type |
+        | len type tag_only |
         aDef isFixed ifTrue:  [len := aDef minLength].
         aDef isVariable ifTrue: [len := (Number readFrom: aDef minLength readStream) - 1].
         aDef isOptional ifTrue: [len := (Number readFrom: aDef minLength readStream) - 2].
@@ -179,13 +179,22 @@ Object subclass: StructCreator [
             aDef minLength printNl.
         ].
 
+
+        tag_only := ''.
+        len <= 0 ifTrue: [
+            len := 0.
+            tag_only := '
+        %1 class >> lengthLength [ ^ 0 ]
+' % {aDef className}.
+        ].
+
         type :=
 'MSGFixedField subclass: %1 [
-        %1 class >> name [ ^ ''%2'' ]
+        %1 class >> parameterName [ ^ ''%2'' ]
         %1 class >> parameterValue [ ^ ISUPConstants par%3 ]
         %1 class >> octalLength [ ^ %4 ]
-        %1 class >> spec [ ^ ''%5'' ]
-]' % {aDef className. aDef commentName. aDef param. len. aDef ref.}.
+        %1 class >> spec [ ^ ''%5'' ]%6
+]' % {aDef className. aDef commentName. aDef param. len. aDef ref. tag_only.}.
 
         self addType: aDef ref struct: type.
     ]
@@ -204,7 +213,7 @@ Object subclass: StructCreator [
 
         type :=
 'MSGVariableField subclass: %1 [
-        %1 class >> name [ ^ ''%2'' ]
+        %1 class >> parameterName [ ^ ''%2'' ]
         %1 class >> parameterValue [ ^ ISUPConstants par%3 ]
         %1 class >> octalLength [ ^ %4 ]
         %1 class >> maxLength [ ^ %5 ]
@@ -220,7 +229,7 @@ Object subclass: StructCreator [
             struct add: '
 ISUPMessage subclass: ISUP%1 [
     ISUP%1 class >> structure [
-        ^ (MSGStructure initWith: ISUPConstants msg%1)' % {structName. }.
+        ^ (self initWith: ISUPConstants msg%1)' % {structName. }.
             ^ true
         ].