I am writing a simple little DSL for specifying constraints on messages, and Have been trying without success for a while to embed XBase expressions into the language.

The Grammar looks like this:

grammar org.xtext.businessrules.BusinessRules with org.eclipse.xtext.xbase.Xbase
//import "http://www.eclipse.org/xtext/xbase/Xbase" as xbase
import "http://www.eclipse.org/xtext/common/JavaVMTypes" as jvmTypes
generate businessRules "http://www.xtext.org/businessrules/BusinessRules"

Start:
    rules+=Constraint*;

Constraint:
    {Constraint}
    'FOR' 'PAYLOAD' payload=PAYLOAD 'ELEMENT' element=ID 'CONSTRAINED BY' constraint=XExpression;



PAYLOAD:
    "SimulationSessionEvents"
    |"stacons"
    |"any"
;



Range:
    'above' min=INT ('below' max=INT)?
    |'below' max=INT ('above' min=INT)?
;

When trying to parse a file such as:

FOR PAYLOAD SimulationSessionEvents ELEMENT matrix CONSTRAINED BY ...

I can't get it to work for ... = any kind of Arithmetic expression, although it works for ...= loop or if expression, or even just a number. As soon as I do something like '-5' or '4-5' it says Couldn't resolve reference to JvmIdentifiableElement '-', even though the Xbase.xtext Grammar looks like it allows these expressions.

I don't think I'm missing any Jars, as it doesn't complain when I run the mwe workflow, but only when trying to parse the input file.

Any help would be much appreciated.