# Lowest precedence.
   bin_op_2 : <leftop: bin_op_1 SUM bin_op_1 >
              { treeify(@{$item[1]}); }

   bin_op_1 : <leftop: term PROD term >
              { treeify(@{$item[1]}); }
   # Highest precedence.

   SUM      : '+' | '-'
   PROD     : '*' | '/' | '\\' | '%'