+@itemx flt64 highest;
+@itemx flt64 lowest;
+The system missing value, the value used for HIGHEST in missing
+values, and the value used for LOWEST in missing values, respectively.
+@xref{System File Format}, for more information.
+
+The SPSSWriter library in PHP, which identifies itself as @code{FOM
+SPSS 1.0.0} in the file header record @code{prod_name} field, writes
+unexpected values to these fields, but it uses the same values
+consistently throughout the rest of the file.
+@end table
+
+@node Multiple Response Sets Records
+@section Multiple Response Sets Records
+
+The system file format has two different types of records that
+represent multiple response sets (@pxref{MRSETS,,,pspp, PSPP Users
+Guide}). The first type of record describes multiple response sets
+that can be understood by SPSS before version 14. The second type of
+record, with a closely related format, is used for multiple dichotomy
+sets that use the CATEGORYLABELS=COUNTEDVALUES feature added in
+version 14.
+
+@example
+/* @r{Header.} */
+int32 rec_type;
+int32 subtype;
+int32 size;
+int32 count;
+
+/* @r{Exactly @code{count} bytes of data.} */
+char mrsets[];
+@end example
+
+@table @code
+@item int32 rec_type;
+Record type. Always set to 7.
+
+@item int32 subtype;
+Record subtype. Set to 7 for records that describe multiple response
+sets understood by SPSS before version 14, or to 19 for records that
+describe dichotomy sets that use the CATEGORYLABELS=COUNTEDVALUES
+feature added in version 14.
+
+@item int32 size;
+The size of each element in the @code{mrsets} member. Always set to 1.
+
+@item int32 count;
+The total number of bytes in @code{mrsets}.
+
+@item char mrsets[];
+Zero or more line feeds (byte 0x0a), followed by a series of multiple
+response sets, each of which consists of the following:
+
+@itemize @bullet
+@item
+The set's name (an identifier that begins with @samp{$}), in mixed
+upper and lower case.
+
+@item
+An equals sign (@samp{=}).
+
+@item
+@samp{C} for a multiple category set, @samp{D} for a multiple
+dichotomy set with CATEGORYLABELS=VARLABELS, or @samp{E} for a
+multiple dichotomy set with CATEGORYLABELS=COUNTEDVALUES.
+
+@item
+For a multiple dichotomy set with CATEGORYLABELS=COUNTEDVALUES, a
+space, followed by a number expressed as decimal digits, followed by a
+space. If LABELSOURCE=VARLABEL was specified on MRSETS, then the
+number is 11; otherwise it is 1.@footnote{This part of the format may
+not be fully understood, because only a single example of each
+possibility has been examined.}
+
+@item
+For either kind of multiple dichotomy set, the counted value, as a
+positive integer count specified as decimal digits, followed by a
+space, followed by as many string bytes as specified in the count. If
+the set contains numeric variables, the string consists of the counted
+integer value expressed as decimal digits. If the set contains string
+variables, the string contains the counted string value. Either way,
+the string may be padded on the right with spaces (older versions of
+SPSS seem to always pad to a width of 8 bytes; newer versions don't).
+
+@item
+A space.
+
+@item
+The multiple response set's label, using the same format as for the
+counted value for multiple dichotomy sets. A string of length 0 means
+that the set does not have a label. A string of length 0 is also
+written if LABELSOURCE=VARLABEL was specified.
+
+@item
+A space.
+
+@item
+The short names of the variables in the set, converted to lowercase,
+each separated from the previous by a single space.
+
+Even though a multiple response set must have at least two variables,
+some system files contain multiple response sets with no variables or
+one variable. The source and meaning of these multiple response sets is
+unknown. (Perhaps they arise from creating a multiple response set
+then deleting all the variables that it contains?)
+
+@item
+One line feed (byte 0x0a). Sometimes multiple, even hundreds, of line
+feeds are present.
+@end itemize
+@end table
+
+Example: Given appropriate variable definitions, consider the
+following MRSETS command:
+
+@example
+MRSETS /MCGROUP NAME=$a LABEL='my mcgroup' VARIABLES=a b c
+ /MDGROUP NAME=$b VARIABLES=g e f d VALUE=55
+ /MDGROUP NAME=$c LABEL='mdgroup #2' VARIABLES=h i j VALUE='Yes'
+ /MDGROUP NAME=$d LABEL='third mdgroup' CATEGORYLABELS=COUNTEDVALUES
+ VARIABLES=k l m VALUE=34
+ /MDGROUP NAME=$e CATEGORYLABELS=COUNTEDVALUES LABELSOURCE=VARLABEL
+ VARIABLES=n o p VALUE='choice'.
+@end example
+
+The above would generate the following multiple response set record of
+subtype 7:
+
+@example
+$a=C 10 my mcgroup a b c
+$b=D2 55 0 g e f d
+$c=D3 Yes 10 mdgroup #2 h i j
+@end example
+
+It would also generate the following multiple response set record with
+subtype 19:
+
+@example
+$d=E 1 2 34 13 third mdgroup k l m
+$e=E 11 6 choice 0 n o p
+@end example