*! version 1.0.1  Fri Apr  8 10:46:03 CDT 1994  (STB20: sg25)
program define xi
	version 3.1
	parse "`*'", parse(" ,:()")
	if "`1'"==":" {
		mac shift
		local i 2
		local xeq "yes"
	}
	else	local i 1

	capture drop I*		/* (crude but necessary) */
	global X__in
	global X__out
	global X__cont
	while "``i''"!="" {
		if upper(substr("``i''",1,2))=="I." {
			if index("``i''","*") { xi_eii ``i'' "*" }
			else if index("``i''","|") { xi_eii ``i'' "|" }
			else	xi_ei ``i''
			local `i' "$S_1"
			if "``i''"=="." { local `i' " " }
		}
		local i=`i'+1
	}
	global X__in
	global X__out
	global X__cont
	if "`xeq'"=="yes" {
		`*'
	}
end
exit

I.myvar		means dummies for myvar, drop the most frequent
I.myvar*this	means continuous interaction (still drop most frequent)
I.myvar*I.that	means dummy interaction.


I.myvar[what]	means dummies for myvar, drop dummy for myvar==what
I.myvar*thatvar means interaction of myvar and thatvar 
I.myvar[val]*thatvar[val] means drop corresponding.
	
12345678
I123_#x#
I12##x##


For I.name*I.name
We try:
	12345
	IrXr_	e.g., IrXr_#_#   for two 1-digit numbers
	Irr_    e.g., Irr_#_## or Irr_##_# for 1 and 2 digit numbers
	Irr     e.g., Irr##_##   for two 2-digit numbers

For I.name*name
	Irr_#

I.abc*I.def

IrXr_ we try, then shorten to 
Irr_

I12345_#
I1234_##
I1234###