S'pose you realize you're in the market for an embedded [DSL] that, let's say,
controls a natural gas pipeline system.  One approach is just to start writing
things you want the language to be able to handle--for example,
   
    set valve(Pittsburgh,18,1349) open
    puts "Flow in critical link is $flow(Valdosta,7,88)."
    if [string equal open ...

But wait!  This is valid Tcl already!  Let's figure out what the output from such
a sequence should be, and we immediately have a case that can be coded directly in
[tcltest].  Now all that remains is to implement (just) enough to pass the test 
we've composed.  This is [TDD] with a vengeance!
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[Category Glossary]