Invalid POU Type for Inheritance

Base Classes and Interfaces can only be used on CLASSes and FUNCTION_BLOCKs, any other POU type is invalid and will result in this error.

Errouneus code example:

INTERFACE interfaceA
    /* ... */
END_INTERFACE

FUNCTION_BLOCK fb
END_FUNCTION_BLOCK

FUNCTION foo EXTENDS fb IMPLEMENTS interfaceA
    /* ... */
END_FUNCTION_BLOCK

In the example above, the POU type of foo should have been CLASS or FUNCTION_BLOCK.