VMake has built-in support for some standard external code generators. Four rules support the YACC and LEX as parser and lexical analyzer generator and AWK and PERL as script language interpreters. The associated rules are Yacc-Target, Lex-Target, Awk-Target and Perl-Target respectively. The usage of external code generators is a little hazard since they may not be available under all configurations, e.g., VMS has none of these tools as standard. To allow development under UNIX and use the output of these tools under VMS, VMake copies the automatically generated code to any release or patch. So rebuilding is not required as long as the source is not modified. During the development of the VISTA project the GNU tools BISON and FLEX as superior replacement for YACC and LEX where adopted to compile under VMake and external rules Bison-Target and Flex-Target have been written (see Appendix D).