I'd like to set up values that display as symbols but evaluate to numbers 
when called with .n().  I think this is similar to how pi and E are 
handled.  In my case it's constants such as the speed of light, and 
Boltzmann's constant.  When I look at the symbolic representation, I'd like 
to see these values represented as symbols (c, k_B), but when I evaluate 
I'd like their true value used without the need for explicit substitution.

Is there an easy way to set up such values?  

Looking through the source, I found Pi and Exp1 defined in 
sympy.core.numbers-- and it looks like each is a custom class.  Is there a 
trick to setting up _as_mpf_val and approximation_interval for physical 
constants such as these?  Does _sage_ need to be implemented if I'm not 
running within Sage?

Thanks--
 Greg

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