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".