expression : INTEGER OP expression
              { main::expression(@item) }
              | VARIABLE OP expression
              { main::expression(@item) }
              | INTEGER
              | VARIABLE
              { $main::VARIABLE{$item{VARIABLE}} ||= 0 }

##</code><code>##

Enter --> a=2 print a
2

Enter --> b=3 print b blah
3

       ERROR (line 1): Invalid instruction: Was expecting assign instruction,
                       or print instruction
Error returned by parsing