program = block '.' .
block =  { declaration } statement .
declaration = constant | variable | function .
constant = CONST constant_definition { ',' constant_definition } ';' .
constant_definition = NAME '=' NUMBER .
variable = 
VAR NAME { ',' NAME } ';' .
function = FUNCTION NAME '(' [ NAME { ',' NAME } ]  ')' block ';' .
expression = [ ( '-' | '+' ) ] term { ( '-' | '+' ) term } .
term = factor { ( '*' | '/' ) factor } .
factor = NAME '(' [ expression { ',' expression } ] ')'
  | NUMBER | NAME | '(' expression ')' .
statement = [ NAME ':=' expression
  | BEGIN statement { ';' statement } END
  | IF condition THEN statement
  | WHILE condition DO statement
  | RETURN expression
  | WRITE expression ] .
condition = ODD expression
  | expression ( '=' | '<>' | '<' | '<=' | '>' | '>=' ) expression .