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*    XSPEC	'selectors' SF package description for PHA files       *
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    This file contains a description of the PHA file 'selectors' package.
It is not directly used by XSPEC, currently, but could contain useful
information of interest to other programs. 

    The 'selectors' package contains 0 or more auxiliary records which
can be used to reconstruct what criteria were used to select the data. 
The header contains a single item.
	AND	L1	If true, all the top level criteria must be
			satisfied.  If false at least one of the top
			level criteria must be satisfied.
If there are 0 auxiliary records to be satisfied then by definition all 
data was acceptable.  Each auxiliary record is 80 bytes consisting of:
	DEPTH	I2	Allowing for compound selectors. Should be zero
			for all the top level criteria. If a particulor
			selector is to be treated as a compound 
			criterion, depth should be incrimented by one 
			and the begin/end flag should be set to TRUE 
			(following).
	PAREN	L1	If true, then it is the beginning or end of a 
			compound criterion.  A `close parenthesis' (end 
			of compound) can be found by looking for the next
			aux. record with the same DEPTH and PAREN = .TRUE.
			This allows for skipping over non-necessary 
			criteria evaluations in a particular compound 
			criterion (e.g. a false one has been found for an 
			AND group or a true one has been found for a 
			NOTAND (or OR) group). If all the criteria are top
			level there is no need for a close parenthesis.
	NOT	L1	For simple criteria and the close parenthesis 
			record of a compound criterion, if TRUE, then the 
			data point must be outside the indicated range of 
			MIN and MAX, if FALSE it must be within the range 
			of MIN and MAX. As a special case on the 'open 
			parenthesis' if the NOT flag is true, it indicates 
			that the compound criterion that follows is an AND 
			criterion, if false, an OR.
	MIN	r8	The lower limit of the evaluation (should be <=MAX)
			(actually there is no reason per se that these be 
			real*8 or even reals. These values should be passed
			to a specific routine (determined by TYPE, see 
			below) which then uses the information passed by 
			the 16 bytes of MIN,MAX to determine whether the 
			criterion is satisfied. It could even be a flag 
			register to be or'd with, or a set of 16 selector 
			characters, or..)
	MAX	r8	The upper limit of the evaluation
	TYPE	c60	A character string that contains the information 
			needed to see what actually was tested.  Examples
			of such strings might be
			"time     |  279.000"
			"phase    |  1 20    718.231789 0.19871 "
			"intensity|  5 24 20 same 0.0 "
The TYPE string is divided into two parts, a unique identifier and then
some useful numbers that might be used in conjunction with MIN and MAX 
above to determine is a particular criterion is satisfied.  The two parts 
are separated by a verticle bar "|" character.  The following are the 
formats for the particular selector types:

	"time" is satisfied if the beginning of an integration is within
the range given by MIN and MAX.  The useful number is an optional <epoch>
which determines the zero point of the time scale (or alternatively the
epoch input in the `source info' package can be used).

	"phase" - The time of the beginning of the integration (or the
midpoint or whatever) falls into a particular phase bin of a folded light
curve.  MIN and MAX are the lower and upper limits of the allowed phase
range.  The auxiliary numbers are <phase bin> <total no. of bins> 
<phase zero> and <fold period>.

	"intensity" - The data was taken during a period where the source
intensity fell into a particular range given by MIN and MAX.  The 
additional numbers are <lower channel range> <upper channel range> <no of 
integrations> <source detector> <background subtracted>.  If the <source 
detector> is the string "same" then it the same detector's data were used 
in accumulating the PHA file.  If <source detector> includes blanks and 
special characters it should be inclosed in a double quote.

	Additional TYPEs can be defined.