java - Antlr4 doesn't find any viable alternative for an input string on a simple grammar -

i'm newbie in antlr , have create grammar parse rules , formulas specified users in project. i've been written hardest 1 contain conditional statement already, second 1 easier first one, no viable alternative input. grammar simple grammar demonstrated below:

grammar pmntidntfrtmpexp;  start     :   statement      |   statement start     ;  statement     :   assignment     ;  assignment     :   id '=' addstmt     ;  addstmt     :   addstmt '+' mulstmt     |   addstmt '-' mulstmt     |   mulstmt     ;   mulstmt     :   mulstmt '*' terminal     |   mulstmt '/' terminal     |   mulstmt '^' terminal     |   terminal     ;  terminal     :   '('addstmt')'     |   id     |   number     ;  id     :   '@fild'(digit)+     ;  number     :   digit+ ('.' digit+)?     ;  digit     :   '0'..'9'     ;  ws       :   [ \t\r\n]+  ->  skip      ; 

but when wanted parse simple string assignment below

@fild76 = @fild12 + @fild43 

i came error

line 1:27 no viable alternative @ input '@fild76=@fild12+@fild43' 

i'll wonder if body me error.

i not know why grammar fails, seems first rule breaks it:

replacing:

start   :   statement    |   statement start   ; 

with syntactically equivalent

start   :   statement+   ; 

does not emit error