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2.4.7 The IF/THEN/ELSE Instruction
     IF expr [;] THEN [;] statement
                             [ ELSE [;] statement ]

This is a normal if-construct. First the boolean expression expr
is evaluated, and its value must be either 0 or 1 (everything else
is a syntax error which raises SYNTAX condition number {34}).
Then, the statement following either THEN or ELSE is executed,
depending on whether expr was 1 or 0, respectively.

Note that there must come a statement after THEN and ELSE.  It is
not allowed to put just a null-clause (i.e. a comment or a label)
there.  If you want the THEN or ELSE part to be empty, use the NOP
instruction. Also note that you can not directly put more than one
statement after THEN or ELSE; you have to package them in a DO-END
pair to make them a single, conceptual statement.

After THEN, after ELSE, and before THEN, you might put one or more
clause delimiters (newlines or semicolons), but these are not
required. Also, the ELSE part is not required either, in which
case no code is executed if expr is false (evaluates to 0). Note
that there must also be a statement separator before ELSE, since
the that statement must be terminated. This also applies to the
statement after ELSE.  However, since statement includes a
trailing clause delimiter itself, this is not explicitly shown in
the syntax diagram.

Example: Dangling ELSE

Note the case of the “dangling” ELSE.  If an ELSE part can
correctly be thought of as belonging to more than one IF/THEN
instruction pair, it will be parsed as belonging to the closest
(i.e. innermost) IF instruction:

     parse pull foo bar
     if foo then
          if bar then
               say ‘foo and bar are true’
               say ‘one or both are false’

In this code, the ELSE instruction is nested to the innermost IF,
i.e. to IF BAR THEN.