Do not crash when checking semantics of undeclared symbolic_variable.
NOTE: this update may have broken the printing of error messages
during semantic analysis of IL code.
IEC 61131-3 Annex F Examples (informative)
This directory contains example code take from:
Annex F of Draft 2nd edition of the IEC 61131-3 standard (1998)
"PROGRAMMABLE CONTROLLERS - PROGRAMMING LANGUAGES"