The following is the Xtext grammar for my DSL.

Model:
  variableTypes=VariableTypes predicateTypes=PredicateTypes variableDeclarations=
  VariableDeclarations rules=Rules;

VariableType:
  name=ID;

VariableTypes:
  'var types' (variableTypes+=VariableType)+;

PredicateTypes:
  'predicate types' (predicateTypes+=PredicateType)+;

PredicateType:
  name=ID '(' (variableTypes+=[VariableType|ID])+ ')';

VariableDeclarations:
  'vars' (variableDeclarations+=VariableDeclaration)+;

VariableDeclaration:
  name=ID ':' type=[VariableType|ID];

Rules:
  'rules' (rules+=Rule)+;

Rule:
  head=Head ':-' body=Body;

Head:
  predicate=Predicate;

Body:
  (predicates+=Predicate)+;

Predicate:
  predicateType=[PredicateType|ID] '(' (terms+=Term)+ ')';

Term:
  variable=Variable;

Variable:
  variableDeclaration=[VariableDeclaration|ID];

terminal WS:
  (' ' | '\t' | '\r' | '\n' | ',')+;

And, the following is a program in the above DSL.

var types
  Node

predicate types
  Edge(Node, Node)
  Path(Node, Node)

vars
  x : Node
  y : Node
  z : Node

rules
  Path(x, y) :- Edge(x, y)
  Path(x, y) :- Path(x, z) Path(z, y)

When I used the generated Switch class to traverse the EMF object model corresponding to the above program, I realized that the nodes are not linked together properly. For example, the getPredicateType() method on a Predicate node returns null. Having read the Xtext user's guide, my impression is that the Xtext default linking semantics should work for my DSL. But, for some reason, the AST nodes of my DSL don't get linked together properly. Can anyone help me in diagnosing this problem?