Program   = { Statement } ;
Statement = { Assignment
            | IfStatement
            | WhileStatement
            | PrintStatement
            | BlockStatement } ;

Assignment  = Identifier
            , '='
            , ExpressionTwo
            , ';' ;
IfStatement = 'if'
            , ExpressionTwo
            , BlockStatement
            , [ 'else'
              , BlockStatement ] ;

(* #[allow(unused)] *)
Unused = 'Hello' ;

WhileStatement = 'while'
               , ExpressionTwo
               , BlockStatement ;

PrintStatement = 'print'
               , ExpressionTwo
               , ';' ;

BlockStatement = '{'
               , { Statement }
               , '}' ;

Identifier = [ 'a' - 'z'
             | 'A' - 'Z' ]
           , { [ 'a' - 'z'
               | 'A' - 'Z'
               | '0' - '9' ] } ;

(* #[allow(unused)] *)
NotUsed = 'Hello' ;

ExpressionTwo = Term
              | ExpressionTwo
              , '+'
              , Term ;
Term          = Factor
              | Term
              , '*'
              , Factor ;
Factor        = Identifier
              | Number
              | '('
              , ExpressionTwo
              , ')' ;
Number        = [ '0' - '9' ]
              , { [ '0' - '9' ] } ;