IBM SC34-5764-01 - Manual (Page 171)

NOP
Purpose
NOP ;
NOP is a dummy instruction that has no effect. It can be useful as the target of a THEN or ELSE clause:
Example:
Select
when a=c then nop /* Do nothing */
when a>c then say 'A > C'
otherwise
say 'A < C'
end
Note: Putting an extra semicolon instead of the NOP would merely insert a null clause, which would be
ignored. The second WHEN clause would be seen as the first instruction expected after the THEN,
and would, therefore, be treated as a syntax error. NOP is a true instruction, however, and is,
therefore, a valid target for the THEN clause.
NOP
Chapter 13. Keyword Instructions 149