2.5.4.15 SET Command
  The SET command specifies defaults for compiler command
  qualifiers. For example:
  SET /SINGLE
  Ready

  This command makes /SINGLE the default for the
  COMPILE or RUN command, thereby making SINGLE
  the default data type for all untyped values. Entering the
  SET command with no arguments resets the defaults to their
  state when you entered the BASIC environment.

  For a full list of options, see the COMPILE command.