Macros: trouble defining them

I defined the following macro:

tg: "{\\mathop{\\mathrm{tg}}\\nolimits}",

the problem is that if I use $\tg x$, then there is no space between tg and x.

However if I write straightforward $\mathop{\mathrm{tg}}\nolimits x$, then there is a space between them :S

Has anyone any idea what causes this problem, and how can I fix it?

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The problem is the extra braces around the macro definition, which means that $\tg x$ is really ${\mathop{\mathrm{tg}}\nolimits} x$ rather than $\mathop{\rm{tg}}\nolimits x$ as you desire. So define

tg: "\\mathop{\\mathrm{tg}}\\nolimits"

instead.

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Thanks :)