anchor.diff:  Each statement which contains a line number (GOTO, GOSUB, IF, ON/GOTO) is modified to use the corresponding new line number.
   
   RENUMBER also adjusts breakpoints, if any are set.
  -
  -RUN
  -~~~
  -
  -Runs the current program.
  -
  -Syntax:	RUN
  -
  -Program execution occurs in two phases.
  -The first phase checks each statement for errors and executes some statements.
  -Any error detected at this stage will halt execution.
  -An error may be a syntax error or another error.
  -GOTO and GOSUB statements with undefined targets are examples of errors.
  -
  -Statements executed in this first phase are the DATA, FILES, and DEF statements.
  -They are executed only once in a program, even if the path of execution travels to them multiple times.
  -
  -The second phase executes statements, starting with the lowest-numbered statement and following the path of execution.
  -
  -SAVE
  -~~~~
  -
  -Saves the current program to disk.
  -Will overwrite an existing file without prompting.
  -
  -Syntax:	SAVE filename
  -
  -The 'filename' argument must be enclosed in quotes.
   
   TOKENS
   ~~~~~~
  
positive.code: They have no line number.
  They are available only in "run" mode.
  
  BREAK
  ~~~~~
  
  Sets, clears, or displays breakpoints.
  
  Syntax: BREAK line-number
  Syntax: BREAK -line-number
  Syntax: BREAK
  
  To set a breakpoint, type 'BREAK' followed by a line number.
  To clear a breakpoint, type 'BREAK' followed by a negative line number.
  To display breakpoints, type 'BREAK' with no arguments.
  
  CROSSREF
  ~~~~~~~~
  
  Lists the numeric constants, text constants, functions, and variables used in the program, along with the line numbers on which they appear.
  
  DELETE
  ~~~~~~
  
  Removes lines from the program.
  
  My experience with other BASIC interpreters is that lines can be deleted by simply typing a line number and pressing RETURN.
  (That is, entering an "empty" line with line number and nothing else.)
  That technique does not work with BASIC-1973, as BASIC-1973 allows empty lines to be part of a program.
  
  So how to remove a line from a program?
  Replacing a line with an empty line is possible, and certainly makes the offending line "go away" from execution, but what if we want to really remove a line?
  
  That's what the DELETE command does.
  
  DELETE uses the same specification as the LIST command.
  
  Syntax:	DELETE [line specification]
  
  If the list specification is a single line, it is deleted immediately.
  If the list specification is a range, the lines are displayed and the user must confirm the operation.
  
  The DELETE command with no specification (implying the entire program) does not delete the program but does nothing.
  (To delete the entire program, use the NEW command.)
  
  LIST
  ~~~~
  
negative.code: |LIST 100+ |Lists line 100 and the next 20 lines.
  |==========
  
  RUN
  ~~~
  
  Runs the current program.
  
  Syntax:	RUN
  
  Program execution occurs in two phases.
  The first phase checks each statement for errors and executes some statements.
  Any error detected at this stage will halt execution.
  An error may be a syntax error or another error.
  GOTO and GOSUB statements with undefined targets are examples of errors.
  
  Statements executed in this first phase are the DATA, FILES, and DEF statements.
  They are executed only once in a program, even if the path of execution travels to them multiple times.
  
  The second phase executes statements, starting with the lowest-numbered statement and following the path of execution.
  
  BREAK
  ~~~~~
  
  Sets, clears, or displays breakpoints.
  
  Syntax: BREAK line-number
  Syntax: BREAK -line-number
  Syntax: BREAK
  
  To set a breakpoint, type 'BREAK' followed by a line number.
  To clear a breakpoint, type 'BREAK' followed by a negative line number.
  To display breakpoints, type 'BREAK' with no arguments.
  
  CROSSREF
  ~~~~~~~~
  
  Lists the numeric constants, text constants, functions, and variables used in the program, along with the line numbers on which they appear.
  
  DELETE
  ~~~~~~
  
  Removes lines from the program.
  
  My experience with other BASIC interpreters is that lines can be deleted by simply typing a line number and pressing RETURN.
  (That is, entering an "empty" line with line number and nothing else.)
  That technique does not work with BASIC-1973, as BASIC-1973 allows empty lines to be part of a program.
  
  So how to remove a line from a program?
  Replacing a line with an empty line is possible, and certainly makes the offending line "go away" from execution, but what if we want to really remove a line?
  
  That's what the DELETE command does.
  
  DELETE uses the same specification as the LIST command.
  
  Syntax:	DELETE [line specification]
  
  If the list specification is a single line, it is deleted immediately.
  If the list specification is a range, the lines are displayed and the user must confirm the operation.
  
  The DELETE command with no specification (implying the entire program) does not delete the program but does nothing.
  (To delete the entire program, use the NEW command.)
  
  PRETTY
  ~~~~~~
  