The rule 'BlockType' may be consumed without object instantiation. Add an action to ensure object creation, e.g. '{BlockType}'.

grammar org.thisAndThat.dsl.SbeDsl with org.eclipse.xtext.common.Terminals

generate thatThatDsl "http://www.thisAndThat.org/dsl/thatThatDsl"

Specification:
    As = TheAs Bs=TheBs;

TheAs:
    'theAs' '{' (theAs+=A)+ '}';

TheBs:
    'theBs' '{'(theBs+=B)+ '}';


A:
    'a' name=Name;

B:
    'b' name=Name '{'
    block=BlockType
    '}';

BlockType:  {BlockType}
    (thisFields+=This)* | (thatFields+=That)*;

This:
    'this' name=Name ':' type=[A];

That:
    'that' name=Name ':' type=[A];

Name:
    ID;

BlockType: {BlockType}
    (thisFields+=This)* | (thatFields+=That)+;

BlockType:  {BlockType}
    (thisFields+=This)+ | (thatFields+=That)*;

B:
    'b' name=Name 
    block=BlockType
    ;

BlockType: {BlockType} 
   '{'  (thisFields+=This)* | (thatFields+=That)* '}';

B:
    'b' name=Name '{'
    block=BlockType
    '}';

BlockType:  
   {BlockType} (thisFields+=This)* |{BlockType} (thatFields+=That)* ;

BlockType: {BlockType}
    thisList =ThisList?
    thatList = ThatList?;

ThisList:
    (thisFields+=This)+
;

ThatList:
    (thatFields+=That)+
;