^TI^'
^TITW^history 
^TITW^change 
^TIS^newValue 
^TIS^eventSpec 
^TI^' Command
^P^
Replaces the value recorded for an event with 
^S^newValue
. The optional 
^S^eventSpec
 specifies the event to replace, and 
defaults to the current event (
^I^not
 event -1) if not explicitly given. 
This command is intended for use in commands that 
implement new forms of history substitution and wish to 
replace the current event (which invokes the substitution) 
with the command created through substitution. The return value 
is an empty string. 
