if then else returns error

[PaulSnow]

ifstatement ::=

     IF bexpr:b THEN block:blk1 ENDIF                     {: RESULT = "{ "+blk1+"} "             +b+ "if "    ; :} |                      
     IF bexpr:b THEN block:blk1 ELSE block:blk2 ENDIF     {: RESULT = "{ "+blk1+"} { "+blk2+"} " +b+ "ifelse "; :} ;