+
+@example
+datum := int64[index] 00? value /* @r{version 1} */
+datum := int64[index] value /* @r{version 3} */
+@end example
+
+The format of a datum varies slightly from version 1 to version 3: in
+version 1 it allows for an extra optional 00 byte.
+
+A datum consists of an index and a value. Suppose there are @math{d}
+dimensions and dimension @math{i} for @math{0 \le i < d} has
+@math{n_i} categories. Consider the datum at coordinates @math{x_i}
+for @math{0 \le i < d}; note that @math{0 \le x_i < n_i}. Then the
+index is calculated by the following algorithm:
+
+@display
+let index = 0
+for each @math{i} from 0 to @math{d - 1}:
+ index = @math{n_i \times} index + @math{x_i}
+@end display
+
+For example, suppose there are 3 dimensions with 3, 4, and 5
+categories, respectively. The datum at coordinates (1, 2, 3) has
+index @math{5 \times (4 \times (3 \times 0 + 1) + 2) + 3 = 33}.
+
+@example
+value := 00? 00? 00? 00? raw-value
+raw-value := 01 opt-value int32[format] double
+ | 02 opt-value int32[format] double string[varname] string[vallab]
+ (01 | 02 | 03)
+ | 03 string[local] opt-value string[id] string[c] (00 | 01)
+ | 04 opt-value int32[format] string[vallab] string[varname]
+ (01 | 02 | 03) string[vallab]
+ | 05 opt-value string[varname] string[varlabel] (01 | 02 | 03)
+ | opt-value string[format] int32[n-substs] substitution*[n-substs]
+substitution := i0 value
+ | int32[x] value*[x + 1] /* @r{x > 0} */
+opt-value := 31 i0 (i0 | i1 string) opt-value-i0-v1 /* @r{version 1} */
+ | 31 i0 (i0 | i1 string) opt-value-i0-v3 /* @r{version 3} */
+ | 31 i1 int32[footnote-number] nested-string
+ | 31 i2 (00 | 01 | 02) 00 (i1 | i2 | i3) nested-string
+ | 31 i3 00 00 01 00 i2 nested-string
+ | 58
+opt-value-i0-v1 := 00 (i1 | i2) 00 00 int32 00 00
+opt-value-i0-v3 := count(counted-string
+ (58 | 31 style)
+ (58
+ | 31 i0 i0 i0 i0 01 00 (01 | 02 | 08)
+ 00 08 00 0a 00))
+
+style := 01? 00? 00? 00? 01 string[fgcolor] string[bgcolor] string[font] byte
+nested-string := 00 00 count(counted-string (58 | 31 style) 58)
+counted-string := count((i0 (58 | 31 string))?)
+@end example