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Execute a command from the operating system, or create a subshell. If no
argument is given, start a subshell by running the default user shell.
The subshell can be terminated by typing 'exit' or 'bye' or 'logout',
depending on system version, and controls return to SIC in this case.
If an argument is given, executes the command in a subshell.
Note that Unix environment variables cannot be defined in such a way,
since it is a subshell. In particular, use command SIC DIRECTORY to
change your working directory.
System commands can also be executed directly from the SIC level using
the $ token. "Command" must be a valid operating system command in the
default shell of the user.