-It is incorrect to have records with duplicate BY values in table lookup
-files.
-
-Any number of FILE and TABLE subcommands may be specified. Each
-instance of FILE or TABLE can be followed by DROP, KEEP, and/or RENAME
-subcommands. These take the same form as the corresponding subcommands
-of @cmd{GET} (@pxref{GET}), and perform the same functions.
+If a table lookup file contains more than one case with a given set of
+BY variables, only the first case is used.
+
+Any number of FILE and TABLE subcommands may be specified.
+Ordinarily, at least two FILE subcommands, or one FILE and at least
+one TABLE, should be specified. Each instance of FILE or TABLE can be
+followed by any sequence of RENAME subcommands. These have the same
+form and meaning as the corresponding subcommands of @cmd{GET}
+(@pxref{GET}), but apply only to variables in the given file.
+
+Each FILE or TABLE may optionally be followed by an IN subcommand,
+which creates a numeric variable with the specified name and format
+F1.0. The IN variable takes value 1 in a case if the given file
+contributed a row to the merged file, 0 otherwise. The DROP, KEEP,
+and RENAME subcommands do not affect IN variables.
+
+When more than one FILE or TABLE contains a variable with a given
+name, those variables must all have the same type (numeric or string)
+and, for string variables, the same width. This rules applies to
+variable names after renaming with RENAME; thus, RENAME can be used to
+resolve conflicts.
+
+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.