binary_op     : '(' subexpression (op subexpression {[\@item[1..2]]} )(s?) ')'
                { [  \$item[2], map { \@\$_ } \@{\$item[3]} ] }

##</code><code>##

binary_op     : '(' subexpression (op subexpression {[@item[1..2]]})(s?) ')'
                { [ $item[2], map { @$_ } @{$item[3]} ] }