program = block "."

block = ["const" ident "=" number {"," ident "=" number} ";"]
        ["var" ident {"," ident} ";"]
        { "procedure" ident ";" block ";" }
        statement .

statement = [ ident ":=" expression
    | "call" ident 
    | "?" ident
    | "!" expression 
    | "begin" statement {";" statement } "end" 
    | "if" condition "then" statement 
    | "while" condition "do" statement ] .

condition = "odd" expression
    | expression ("="|"#"|"<"|"<="|">"|">=") expression .

expression = [ "+"|"-"] term { ("+"|"-") term} .

term = factor {("*"|"/") factor} .

factor = ident | number | "(" expression ")" .