17.4.4 %PRINT Directive
  The %PRINT directive allows you to insert a message into
  your source code that the BASIC compiler displays at compile
  time.

  The text must be a quoted string literal. This information is
  displayed to SYS$ERROR and in the compilation listing if one
  is being created. BASIC prints the message specified as soon
  as it encounters a %PRINT directive. See Section 17.4.5 for
  an example of using %PRINT.