  IF

   IF <expression> THEN
       <body>
   [ELIF]
       <body>
   [ELSE]
       [body]
   ENDIF | END IF | FI

   Type: statement

   Execute <body> if <expression> is true. If <expression> is not true then
   run the optional ELSE body. Multiple IF's can be written with ELIF. The
   IF construction should end with ENDIF or END IF or FI. Example:

   a = 0
   IF a > 10 THEN
       PRINT "This is strange:"
       PRINT "a is bigger than 10"
   ELSE
       PRINT "a is smaller than 10"
   END IF

   If only one function or statement has to be executed, then the
   if-statement also can be used without a body. For example:

   IF age > 18 THEN PRINT "You are an adult"
   ELSE INPUT "Your age: ", age

   Use with care as nested IF/THEN statements using one function may
   confuse the parser.

