You can set a breakpoint on any source line that has a toggle button to
  its left in the source view.  These are the lines for which the
  compiler has generated executable code (routine declarations,
  assignment statements, and so on).

  To set a breakpoint on a source line:

  1.  Find the source line on which you want to set a breakpoint.

  2.  Click on the toggle button to the left of that line.  (The
      breakpoint is set when the toggle button is filled in.) The
      breakpoint is set at the start of the source line--that is, on the
      first machine-code instruction associated with that line.