#ifdef SYMBOL_100
// SYMBOL_100 is defined.
#elif defined (SYMBOL_200)
// SYMBOL_200 is defined; but SYMBOL_100 is not.
#endif

-- Let's try some text replacements here. What about commenting out the
   grayed lines?

   1. Expand the Search Bar (at the bottom), possibly with the menu item
      [Search | Replace].
   2. Put the pattern ".*" (without the quotes) in the [Search] edit box.
      Make sure the [RE] check box is checked.
   3. Put the pattern "//{c}&" (without the quotes) in the [Replace Pattern]
      edit box.
   4. Click the button [Replace All].
   5. Hide the grayed lines by unchecking the [Show Complete] check box on
      the Tool Bar.
   6. Put the pattern "//{c}\(.*\)" (without the quotes) in the [Search]
      edit box.
   7. Put the pattern "\1" (without the quotes) in the [Replace Pattern]
      edit box.
   8. Click the button [Replace All].
   9. Show the grayed lines by checking the [Show Complete] check box on
      the Tool Bar.

   Now, all grayed lines are commented out. If you want to "undo" the
   commenting, replace again. How will you do this?

