5E. conditional statement (IF)
------------------------------
IF, THEN, ELSE, ELSEIF, ENDIF

syntax:		IF <exp> THEN <statement> [ ELSE <statement> ]
or:		IF <exp>
                  <statements>
		[ ELSEIF <exp>           /* multiple elseifs may occur */
                  <statements> ]
		[ ELSE ]
                  <statements>
		ENDIF

builds a conditional block. Note that there are two general forms of
this statement, a single-line and a multiple-line version.