XText cross referencing - eclipse

Question

I have written following grammar
Model:
package = PackageDec?
greetings+=Greeting*
usage+=Usage* ;
PackageDec:
'package' name=QualifiedName ;
Greeting:
'greet' name=ID '{' ops += Operation* '}' ;
Operation:
'op' name=ID ('(' ')' '{' '}')? ;
QualifiedName:
ID ('.' ID)*;
Usage:
'use';
With above i can write following script.
package p1.p2
greet G1 {op f1 op f2 }
Now i need to write something like this:
package p1.p2
greet G1 {op f1 op f2 op f3}
use p1.p2.G1.f1
use p1.p2.G1
use p1.p2.G1.f3
To support that i changed Usage RULE like this
Usage:
'use' head=[Greet|QualifiedName] =>('.' tail=[Operation])?
However when i generate xtext artifacts it is complaining about multiple alternatives.
Please let me know how to write correct grammar rule for this.

Answer 1

This is because QualifiedName consumes dots (.). Adding ('.' ...)? makes two alternatives. Consider input
a.b.c
This could be parsed as
head="a" tail = "b.c"
head="a.b" tail = "c"
If I understand your intention of using predicate => right, than you just have to replace
head=[Greet|QualifiedName]
with
head=[Greet]
In this case however you will not be able to parse references with dots.
As a solution I would recommend to substitute your dot with some other character. For example with colon:
Usage:
'use' head=[Greet|QualifiedName] (':' tail=[Operation])?