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The commands in Xselect consist of a command name, and a group of parameters. The parameters have two seperate qualifiers, they can be queried or hidden, and learned or fixed.
The queried parameters will be prompted for if not entered on the command line, and can be entered (provided the correct order is maintained) without their parameter name. The hidden parameters must be entered in the form name=value and will not be prompted for. Rather a default is provided in the Xselect parameter file.
The learned parameters will have their defaults set by the last entered value, whereas the fixed parameters will revert to their original defaults when no value is specified for them.
The nature of any given parameter can be determined by the LPARM command, or by typing the command name followed by a ?.
The parameter file (xselect.par) contains all the Xselect parameters and their defaults, and maintains the state of Xselect between invocations. It is managed by APE following the standard HEASoft conventions: a system copy is kept with the installed software, and a local copy is made in your pfiles directory (normally $HOME/pfiles) the first time Xselect is used. This local copy is the one that will be used afterwards. If you want to reset the parameters in Xselect to the shipped state, just remove xselect.par from your pfiles directory, and you will receive a fresh copy the next time you start Xselect.
The convention for giving the syntax of commands that is used in this document is as follows:
xsel > comm <par1name> <par2name> [par3name] [par4name]
Here, comm is the command, and parXname is the name of the parameter.
The parameters in arrow-brackets 
will be prompted for if not given
on the command line. The parameters in [] are hidden. Thus, in the
above example, entering comm to the XSELECT prompt will result in the
following:
xsel > comm > Value of par1name? >[default1] > Value of par2name? >[default2] xsel >
where the prompting messages are, of course, tailored to the situation
and the user has to either enter the parameters when asked or type
CR to get the defaults. To change the hidden parameters, the user
must type
xsel > comm par3name=value3 par4name=value4
Order does not matter for hidden parameters.
All commands and parameter names can be abbreviated to the shortest unique string, and are not case sensitive. The same is true for the values of the verb_what parameters. I.e. the following all produce the same result:
xsel > show obscat
xsel > show obs
xsel > sh OBS ...