# ...
   expression : INTEGER OP expression
              { main::expression(@item) }
              | VARIABLE OP expression
              { main::expression(@item) }
              | INTEGER
              | VARIABLE
              <defer: $main::VARIABLE{$item{VARIABLE}} ||= 0 >
              { $main::VARIABLE{$item{VARIABLE}}}
              
   print_instruction  : /print/i expression
                      <defer: print $item{expression}."\n" >
   assign_instruction : VARIABLE "=" expression
                      <defer: $main::VARIABLE{$item{VARIABLE}} = $item{expression} >
# ...