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 /build/reproducible-path/macaulay2-1.25.06+ds/M2/Macaulay2/packages/Brackets.m2:667:0.