a = b_R;
Interpreting bracket polynomials as functions on $\Lambda^d (V)$ (see GCExpression ^ GCExpression for notation), we may convert an extensor $a_1 \wedge \cdots \wedge a_d$ to the bracket $[a_1, \ldots , a_d].$
The source of this document is in Brackets.m2:667:0.