program          : [[statement | expression] Delimiter ? ]*;
statement        : [declaration | definition];
declaration      : extern prototype;
definition       : def prototype expression;
prototype        : Ident OpeningParenthesis [Ident Comma ?]* ClosingParenthesis;
expression       : [primary_expr (Op primary_expr)*];
primary_expr     : [Ident | Number | call_expr | parenthesis_expr];
call_expr        : Ident OpeningParenthesis [expression Comma ?]* ClosingParenthesis;
parenthesis_expr : OpeningParenthesis expression ClosingParenthesis;