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Please enable AMSsymbols extension for MathJax

I typed the command \subsetneq in an answer a few moments ago and it didn't render : the output just reproduced the command in red. I also tried \subsetneqq, \varsubsetneq and \varsubsetneqq, but to no avail. Finally I resorted to the clumsy parenthetical remark "strict inclusion". Could some maven please explain what I should do? (I apologize if this question has already been asked.)