When checking semantics of expressions, skip semantic checking of data type definitions
(not yet supported). If we don't skip, all subsequent POUs would not be checked.
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"