Use duplicate symtable instead of symtable for enum constant value table (this will later allow us to detect semantic errors in IEC 61131-3 source 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"