Defined in packages/acs-core/utilities-procs.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 ns_write 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