MainRule:
	'{' 'MAIN'? main=Pattern '}' ';'?
;

Pattern : 
	Concat
;

Concat returns Pattern :
	Union ({Concat.left=current} right=Union)*
;

Union returns Pattern:
	And ({Union.left=current} '|' right=And)* 
;

And returns Pattern:
	Primary ({And.left=current} '&' right=Primary)*
;

Primary returns Pattern
	:  {Optional} '(' pattern=Concat ')'
	|  {Multi} '{' pattern=Concat '}'
	|  {Not} '!' pattern=Primary 
	|  {Atomic} pattern=Atomic
    ;

Atomic returns Pattern
	:  {Word} value=STRING
	|  {Feature} value=ID
;

FeatureA"someword"   // what they typed and no error

FeatureA&"someword"   // what they should type

FeatureA  "someword"  // xtext parsed as Concat