Negated inputs in ladder

This test case it part of the test suite proposed for new devices.

Test case ID: R0256
Language: LD

Code:

tests2/t0256.fbd
    <vars name="A" datatype="BOOL" init="TRUE"/>
    <vars name="B" datatype="BOOL"/>
    <vars name="C" datatype="BOOL"/>
    <vars name="D" datatype="BOOL"/>
 (* graphical code omitted in this listing *)