+
+@example
+datum := int64[index] 00? value @r{# Version 1.}
+datum := int64[index] value @r{# Version 3.}
+@end example
+
+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}.
+
+The format of a datum varies slightly from version 1 to version 3, in
+that version 1 has an extra optional 00 byte.