SET SPELLER [program]  -  define a program for the SPELL command
Usually:    /usr/local/bin/ispell

The SPELLER variable defines the spell checker program to be used
  with the SPELL command.

For more information on ispell, type "!man ispell".

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show   spell
