4.1 Overview of the Debugger
  A debugger is a tool to help you locate run-time errors
  quickly. It is used with a program that has already been
  compiled and linked successfully, with no errors reported, but
  that does not run correctly. For example, the output may be
  obviously wrong, the program goes into an infinite loop, or the
  program terminates prematurely. The debugger enables you
  to observe and manipulate the program's execution inter-
  actively, step by step, until you locate the point at which the
  program stopped working correctly.

  The OpenVMS Debugger is a symbolic debugger , which
  means that you can refer to program locations by the symbols
  (names) you used for those locations in your program-the
  names of variables, routines, labels, and so on. You do not
  have to use virtual addresses to refer to memory locations.

  The debugger recognizes the syntax, expressions, data typing,
  and other constructs of BASIC.