st: problem with marksample touse

[Xuan Zhang <xuanzhan@buffalo.edu>]

In the shamhaz ado file, there are several lines like these:

if "`weight'"=="" {
		if trim("``opt2''")=="" qui logit `tmplhs' ``rhsvars'' if `touse' &
(`tmplhs'==0 | (`tmplhs'==1 & L.`tmplhs'==0 & L.`touse'==1))

Question1
I have problem understanding what's meaning of  if `touse'. Is it a
short version of if 'touse'==1?  I mean after the syntax command, what
exactly marksample do to my data? I searched some note on the internet
and one of them says "The marksample touse ensures that only
observations satisfying the if or in clauses are used." but since
there is no if option in the ado code, does that mean that every
observation in my data is marked as 1? really confused.

syntax varlist(ts) [if] [in] [aweight/] [, *]
marksample touse

Question 2
weight is a macro that STATA keeps it for itself, right? I guess in
the first line of the code, if "`weight'"==""  means if there is no
option after the logit command, then quietly logit left hand side
variable with the right hand side variable without the option. But I'm
not sure about that.

Thanks a lot.

Vicky
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/