2.5.4.14 SEQUENCE Command
  The SEQUENCE command automatically generates line
  numbers for input text. After a SEQUENCE command,
  VAX BASIC prompts with a line number and prompts again
  after each source line you enter. If you press Ctrl/Z (either
  in response to the line number prompt or at the end of a
  program line), VAX BASIC stops prompting, and you can
  enter source text in the normal way. If you specify a starting
  line number that already contains a statement, VAX BASIC
  signals ``Attempt to sequence over existing statement'' and
  returns to normal input mode.

  Note that the SEQUENCE command is not allowed on pro-
  grams without line numbers.