Invalid POU Type for Interface Implementation

Interfaces can only be implemented 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 foo IMPLEMENTS interfaceA
    /* ... */
END_FUNCTION_BLOCK

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