How do one modify an expression?

I am using expressions to annotate x and y labels in plots. I start of with
a label as

  xlab <- expression(X)

Note that it is the user that sets this. It could also be something more
complicated as expression(sqrt(R*G)). Can I easily add a log[2] "around"
this expression? I would like to get

  xlab <- expression(log[2](X))

I basically know nothing about expressions.

Thanks

Henrik Bengtsson

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