Bold square brackets

How do I typeset a bold square bracket? I need to typeset it about 3 times thicker than it is (to denote a specific mathematical operator). {\bf[} does not do this. Things like [\![ do not seem to produce good quality. "The Comprehensive LaTeX Symbol List" does not seem to mention an appropriate symbol.

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\usepackage{bm} and then \bm{[}. Maybe this isn't bold enough for you in which case you can use a font with a heavy variant (eg the commercial mathtime set) and \usepackage{mathtime,bm} and then \hm{[}.
Otherwise maybe you can use a different symbol, such as double square bracket \llbracket from \usepackage{stmaryrd}.