1
expr = term , [ inline op prec 1 , epxr ] ;
2
term = value [ "^" expr ] [ "_" expr ] , [ inline op prec 2 , term ] ;
3
value = number | symbol | bracket expr ;
4
bracket expr = "{" , expr , "}" | "(" , expr , ")" | "[" , expr , "]" | "\{" , expr , "\}" ;