Commented by: Kuznetsov Eugene (eugene)

Since 2.5, isql, if set command is invalid ("set 1", for example) get valid options list. In 2.5 RC2 these are
AUTOddl BLOBdisplay BAIL
BULK_INSERT COUNT ECHO
GENERATOR HEADING LIST
NAMES PLAN PLANONLY
ROWCOUNT STATs SQLDA_DISPLAY
SQL STATISTICS TRANSaction
TERMinator TIME WIDTH
WARNINGs WNG

Help set command output these options:
SET
SET AUTOddl
SET BAIL
SET BLOB [ALL|<n>]
SET BLOB
SET COUNT
SET ROWCOUNT [<n>]
SET ECHO
SET HEADING
SET LIST
SET NAMES <csname>
SET PLAN
SET PLANONLY
SET SQL DIALECT <n>
SET STATs
SET TIME
SET TERM <string>
SET WIDTH <col> [<n>]

I think, "help set" should output the list
SET
SET AUTOddl
SET BAIL
SET BLOBdisplay [ALL|<n>]
SET BLOBdisplay
SET COUNT
SET ROWCOUNT [<n>]
SET ECHO
SET HEADING
SET LIST
SET NAMES <csname>
SET PLAN
SET PLANONLY
SET SQLDA_DISPLAY
SET SQL DIALECT <n>
SET STATs
SET TIME
SET TERMinator <string>
SET WIDTH <col> [<n>]
SET WARININGs;
SET WNG;

and, maybe, SET BULK_INSERT as it seems some undocumented isql option.