Defined in tcl/ecommerce-utilities.tcllike decode in sql
Takes the place of an if (or switch) statement -- convenient because it's compact and
you don't have to break out of an append doc_body if you're in one.
args: same order as in sql: first the unknown value, then any number of pairs denoting
if the unknown value is equal to first element of pair, then return second element, then
if the unknown value is not equal to any of the first elements, return the last arg