Incompatible firmware versions are listed in the "requires" tag's
content. Rationale: since compatibility should be the rule and
incompatibility should be the exception, the tag lists incompatible
firmware only.
Incompatibile firmware versions are listed in "firmware" tags, with a
version typically given as regular expression.
The content of each "firmware" is "not-child", if a receiver is
incompatible with a wireless device or "not-parent" if a wireless
device is incompatible with a receiver (currently, only the former
exists at Logitech).
In the former case, all wireless devices must be upgraded (with
a compatible firmware) before the receiver.
In the latter case, the receiver must be upgraded (with a
compatible firmware) before the wireless devices.
Incompatibility (pre-requisite) handling: