Please provide a minimal grammar.
Please solve the obvious ambiguity

A=ID |B=ID

First

grammar org.xtext.example.expressiondsl.ExpressionDSL with org.eclipse.xtext.common.Terminals

generate expressionDSL "http://www.xtext.org/example/expressiondsl/ExpressionDSL"

Model:
statements+=Statement*;

Statement:
VariableDef |
ConstDef |
StructDef |
FunctionDef |
FunctionCallStatement |
VariableAssignment
;

VariableDef:
'var' type=TYPE name=ValidID (options=Dim)? ';'
;

ConstDef:
'val' type=TYPE name=ValidID (options=Dim)? ';'
;

StructDef:
'struct' name=ValidID (options=Dim)? ';'
subFields+=(SubField)*
'endstruct' ';'
;

SubFieldDef:
('subf')? type=TYPE name=ValidID (options=Dim)? ';'
;

SubField:
SubFieldDef | StructDef
;

FunctionDef:
'def' type=TYPE name=ValidID ';'
;

VariableAssignment:
tgtvar=[VariableDef]
op=('=' | '+=' | '-=' | '*=' | '/=' | '**=' )
exp=Expression ';'
;
FunctionCallStatement:
'call'? call=FunctionCall ";"
;

FunctionCall:
=> ref=[FunctionDef]
('('
(params+=Expression (':' params+=Expression)*)?
')')
;

// ArrayExpOption1
//ArrayExp returns Expression:
// Primary (
// => ( {ArrayExp.left=current} '(' arrayDimensions=Expression ')' )
// )?
//;

//ArrayExpOption2
//ArrayExp:
// => ref=[Variable] '(' index=Expression ')'
//;

//ArrayExpOption3
// https://www.eclipse.org/forums/index.php/t/503752/
ArrayExp:
VariableArray |
StructArray |
SubFieldArray
;
VariableArray returns ArrayExp:
{VariableArray} => ref=[VariableDef] '(' arrayDimensions=Expression ')'
;
StructArray returns ArrayExp:
{StructArray} => ref=[StructDef] '(' arrayDimensions=Expression ')'
;
SubFieldArray returns ArrayExp:
{SubFieldArray} => ref=[SubFieldDef] '(' arrayDimensions=Expression ')'
;

DotExpression returns Ref:
=> StructArray ({DotExpression.ref=current} "." (tail=[SubFieldArray] | tail=[StructArray]) )*
;
//--------------------------------------------------------------------------
Expression: Or;
Or returns Expression:
UnaryPlusMinusOrNot (
{Or.left=current} 'OR'
right=And
)*;
UnaryPlusMinusOrNot returns Expression:
FunctionExp |
( ({UnaryPlus} '+' | {UnaryMinus} '-' | {Not} 'NOT') expr=UnaryPlusMinusOrNot )
;
FunctionExp returns Expression:
// ArrayExp |
FunctionCall|
Primary
;
Primary returns Expression:
DotExpression |
ArrayExp |
// QualifiedName |
'(' Expression ')' |
Atomic
;
Atomic returns Expression:
{IntConstant} value=INT |
{StringConstant} value=STRING |
{BooleanConstant} value=BOOL
;
//--------------------------------------------------------------------------
VariableOrConstRef:
VariableDef |
ConstDef |
StructDef |
SubField
;

Variable:
VariableDef |
StructDef |
SubField
;


Dim:
//Dimensions of Array (as in how many elements)
'DIM' '(' arrayDimensions=INT ')'
;

//--------------------------------------------------------------------------
// Helper
// https://dslmeinte.wordpress.com/2014/09/11/ambiguitiy-in-xtext-grammars-part-2/
Named:
VariableDef |
ConstDef |
StructDef |
SubFieldDef |
FunctionDef
;

ValidID:
ID
;

//QualifiedName:
// ArrayExp ({ArrayExp.left=current} '.' tail=ArrayExp)*
//;

terminal TYPE:
'int' | 'char' | 'bool'
;
terminal BOOL:
'true' | 'false'
;

you have no other change unless you introduce a different syntax