nonrandom toolchanger:

    T0 is special and means "no tool".  T0 should not appear in the tool
    table, if T0 is defined there is ignored.


random toolchanger:

    T0 is a normal tool like any other, totally devoid of special meaning.
    In practice T0 is usually(?) used to track an empty pocket in the
    tool changer hardware.

    Tool number -1 is used to indicate "unknown tool".