program    <- _ block '.' _

block      <- const var procedure statement
const      <- ('CONST' _ ident '=' _ number (',' _ ident '=' _ number)* ';' _)?
var        <- ('VAR' _ ident (',' _ ident)* ';' _)?
procedure  <- ('PROCEDURE' _ ident ';' _ block ';' _)*

statement  <- (assignment / call / statements / if / while / out / in)?
assignment <- ident ':=' _ expression
call       <- 'CALL' _ ident
statements <- 'BEGIN' _ statement (';' _ statement )* 'END' _
if         <- 'IF' _ condition 'THEN' _ statement
while      <- 'WHILE' _ condition 'DO' _ statement
out        <- ('out' / 'write' / '!') _ expression
in         <- ('in' / 'read' / '?') _ ident

condition  <- odd / compare
odd        <- 'ODD' _ expression
compare    <- expression compare_op expression
compare_op <- < '=' / '#' / '<=' / '<' / '>=' / '>' > _

expression <- sign term (term_op term)*
sign       <- < [-+]? > _
term_op    <- < [-+] > _

term       <- factor (factor_op factor)*
factor_op  <- < [*/] > _

factor     <- ident / number / '(' _ expression ')' _

ident      <- < [a-z] ([a-z] / [0-9])* > _
number     <- < [0-9]+ > _

~_         <- [ \t\r\n]*