this breaks 4.1's pattern matcher for left-recursion. cleaned up in main branch I believe. try downloading last master and build. CUrrently 4.1 generates:
expr[int _p] : ( {} variable | number | function_def | OPEN_PAREN expr CLOSE_PAREN ) ( {2 >= $_p}? expr )* ;
for that rule. expr ref in loop is expr[0] actually, which isn't right.