SELECT [expression]
   WHEN expression THEN instruction
   WHEN expression THEN instruction
   ...
   [OTHERWISE instructions]
END [SELECT]

If an expression is supplied after SELECT, then this construction is like
a Pascal or Modula `CASE' construction. The expression is compared with
each of the other expressions in turn, until one is found that matches
(using the `=' form of equality). If a match is found, then the
corresponding instruction is executed. If a match is not found, then an
OTHERWISE clause must be specified, or else an error is reported. The
instructions after OTHERWISE will be executed.
  If an expression is not specified after SELECT, then the construction
is a list of guarded commands - that is, each expression is evaluated
until a true one is found, and the corresponding action is taken. Again,
if no expression is true and there is no OTHERWISE then an error is
reported.
  The word SELECT may be placed after the closing END but is optional.
Adding the word SELECT allows easier detection of missing ENDs.
  It is an error for the word WHEN or OTHERWISE to appear anywhere except
immediately inside a SELECT construction.
  Precisely one instruction is required after each WHEN, so NOP should be
used if no action is required. NOP need not be used after OTHERWISE.
  If multiple instructions are required to follow each WHEN condition,
then they should be contained in a DO ... END block.  Multiple
instructions may follow the OTHERWISE keyword.

LOCAL: The optional expression following the SELECT keyword, and the
       optional SELECT following the END keyword are features of PL/1
       and not of standard REXX.