[R] How to join two plotmath type expressions

[Trevor Carey-Smith]

If I have two variables set, one of which holds an expression, for example:

 > label <- 'Temperature'
 > unit <- expression(paste(degree,'C'))

how can I join them together for use in mtext (or equivalent)? The
following is conceptually what I want to do, but will obviously not work:

 > plot(0:1)
 > mtext(paste(label,unit))

Preferably, I would like this to work if one, both or neither of label
and unit are expressions.

Thanks,
Trevor.
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