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 ")" .