$$ in parse.y question(c++)

[Dave Raggett]

In parse.y(gcc/cp/parse.y),I find such code:
expr_no_commas:
| expr_no_commas '+' expr_no_commas
		{ $$ = build_x_binary_op ($2, $$, $3); }

why not { $$ = build_x_binary_op ($2, $1, $3); }??
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