Prime or dash in plotmath().

I've had to struggle a bit to get the prime/dash derivative notation
into a plotmath expression.  The use of

		expression(K'(t))

gives me a syntax error.  The best I've been able to come up with is

		expression(paste(K,"'",(t),sep=""))

Is this the ``accepted'' procedure, or is there a more direct route?

				cheers,

					Rolf Turner
					rolf at math.unb.ca