libboost-signals1.67-dev 1.67.0-17ubuntu8 (amd64 binary) in ubuntu focal

 This package forms part of the Boost C++ Libraries collection.
 .
 Signals represent callbacks with multiple targets, and are also
 called publishers or events in similar systems. Signals are connected
 to some set of slots, which are callback receivers (also called event
 targets or subscribers), which are called when the signal is
 "emitted."