[O] inline constant substitution

[Paul Rudin]

Suppose I have: 

#+CONSTANTS: foo=42

How can I make inline references to a constant on export in regular
text, rather than in a table?

I want to be able to write something like: 

  The value of foo is $foo 

in my org file, and then end up with:

  The value of foo is 42

when I export to LaTeX.

TIA.