TECO has a number of internal mode flags which can be
  set and/or read to modify the standard TECO environment.

  To read the value of a flag, use
      Fx
  where "F" is the name of the flag and "x" is a command
  which accepts a numeric argument.

  To set the value of a flag, use
      xF
  where "F" is the name of a flag and "x" is a number or
  a command which returns a numeric value.