+FILE and TABLE must be specified at the beginning of the command, with
+any RENAME or IN specifications immediately after the corresponding
+FILE or TABLE. These subcommands are followed by BY, DROP, KEEP,
+FIRST, LAST, and MAP.
+
+The BY subcommand specifies a list of variables that are used to match
+cases from each of the files. When TABLE or IN is used, BY is
+required; otherwise, it is optional. When BY is specified, all the
+files named on FILE and TABLE subcommands must be sorted in ascending
+order of the BY variables. Variables belonging to files that are not
+present for the current case are set to the system-missing value for
+numeric variables or spaces for string variables.
+
+The DROP and KEEP subcommands allow variables to be dropped from or
+reordered within the new active file. These subcommands have the same
+form and meaning as the corresponding subcommands of @cmd{GET}
+(@pxref{GET}). They apply to the new active file as a whole, not to
+individual input files. The variable names specified on DROP and KEEP
+are those after any renaming with RENAME.
+
+The optional FIRST and LAST subcommands name variables that @cmd{MATCH
+FILES} adds to the active file. The new variables are numeric with
+print and write format F1.0. The value of the FIRST variable is 1 in
+the first case with a given set of values for the BY variables, and 0
+in other cases. Similarly, the LAST variable is 1 in the last case
+with a given of BY values, and 0 in other cases.