package unknown ?command?
This command supplies a "last resort" command to invoke during package require if no suitable version of a package can be found in the package ifneeded database. If the command argument is supplied, it contains the first part of a command; when the command is invoked during a package require command, Tcl appends two additional arguments giving the desired package name and version. For example, if command is foo bar and later the command package require test 2.4 is invoked, then Tcl will execute the command foo bar test 2.4 to load the package. If no version number is supplied to the package require command, then the version argument for the invoked command will be an empty string. If the package unknown command is invoked without a command argument, then the current package unknown script is returned, or an empty string if there is none. If command is specified as an empty string, then the current package unknown script is removed, if there is one.