Library /sys$common/syshlp/basichelp.hlb Compile time errors, CONCOMSYN  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  FATAL - The /SYNTAX_CHECKING qualifier is in  effect  when  a  program
  line  containing  the %IF, %THEN, %ELSE, or %END %IF lexical directive
  was entered.  Turn off syntax checking before entering a program  line
  containing lexical directives.